future time travel paradox

• Skip to main content
• Keyboard shortcuts for audio player

• LISTEN & FOLLOW
• Apple Podcasts
• Google Podcasts
• Amazon Music
• Amazon Alexa

Your support helps make our show possible and unlocks access to our sponsor-free feed.

Paradox-Free Time Travel Is Theoretically Possible, Researchers Say

Matthew S. Schwartz

A dog dressed as Marty McFly from Back to the Future attends the Tompkins Square Halloween Dog Parade in 2015. New research says time travel might be possible without the problems McFly encountered. Timothy A. Clary/AFP via Getty Images hide caption

A dog dressed as Marty McFly from Back to the Future attends the Tompkins Square Halloween Dog Parade in 2015. New research says time travel might be possible without the problems McFly encountered.

"The past is obdurate," Stephen King wrote in his book about a man who goes back in time to prevent the Kennedy assassination. "It doesn't want to be changed."

Turns out, King might have been on to something.

Countless science fiction tales have explored the paradox of what would happen if you went back in time and did something in the past that endangered the future. Perhaps one of the most famous pop culture examples is in Back to the Future , when Marty McFly goes back in time and accidentally stops his parents from meeting, putting his own existence in jeopardy.

But maybe McFly wasn't in much danger after all. According a new paper from researchers at the University of Queensland, even if time travel were possible, the paradox couldn't actually exist.

Researchers ran the numbers and determined that even if you made a change in the past, the timeline would essentially self-correct, ensuring that whatever happened to send you back in time would still happen.

"Say you traveled in time in an attempt to stop COVID-19's patient zero from being exposed to the virus," University of Queensland scientist Fabio Costa told the university's news service .

"However, if you stopped that individual from becoming infected, that would eliminate the motivation for you to go back and stop the pandemic in the first place," said Costa, who co-authored the paper with honors undergraduate student Germain Tobar.

"This is a paradox — an inconsistency that often leads people to think that time travel cannot occur in our universe."

A variation is known as the "grandfather paradox" — in which a time traveler kills their own grandfather, in the process preventing the time traveler's birth.

The logical paradox has given researchers a headache, in part because according to Einstein's theory of general relativity, "closed timelike curves" are possible, theoretically allowing an observer to travel back in time and interact with their past self — potentially endangering their own existence.

But these researchers say that such a paradox wouldn't necessarily exist, because events would adjust themselves.

Take the coronavirus patient zero example. "You might try and stop patient zero from becoming infected, but in doing so, you would catch the virus and become patient zero, or someone else would," Tobar told the university's news service.

In other words, a time traveler could make changes, but the original outcome would still find a way to happen — maybe not the same way it happened in the first timeline but close enough so that the time traveler would still exist and would still be motivated to go back in time.

"No matter what you did, the salient events would just recalibrate around you," Tobar said.

The paper, "Reversible dynamics with closed time-like curves and freedom of choice," was published last week in the peer-reviewed journal Classical and Quantum Gravity . The findings seem consistent with another time travel study published this summer in the peer-reviewed journal Physical Review Letters. That study found that changes made in the past won't drastically alter the future.

Bestselling science fiction author Blake Crouch, who has written extensively about time travel, said the new study seems to support what certain time travel tropes have posited all along.

"The universe is deterministic and attempts to alter Past Event X are destined to be the forces which bring Past Event X into being," Crouch told NPR via email. "So the future can affect the past. Or maybe time is just an illusion. But I guess it's cool that the math checks out."

• grandfather paradox
• time travel

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

• View all journals
• Explore content
• About the journal
• Publish with us
• Sign up for alerts
• 27 March 2024

The real time-travel paradox was the friends we made along the way

• Rodrigo Culagovski 0

Rodrigo is a Chilean architect, designer and web developer. He currently heads a web development agency and is a researcher and professor at Universidad Católica in Chile. He has published in Dark Matter Presents: Monstrous Futures , Solarpunk Magazine and Future Science Fiction Digest . On Mastodon as @[email protected] . He misses his Commodore 64. Pronouns he/him/él.

You can also search for this author in PubMed   Google Scholar

Illustration: Jacey

You have full access to this article via your institution.

She was taller than me. Prettier and with better muscle tone. Shinier hair and perfect skin and teeth. Which was odd because she claimed she was me — from the future.

“Mmmmf!” I said.

“Sorry about the gag. Let me loosen it.”

“What the hell!? You’re here to kill me — won’t that kill you, too?”

She rolled her eyes. “No, it didn’t. I’m here, aren’t I?”

I scoffed. “I might not be a time-travelling assassin supermodel —”

“Yet,” she interjected with a smile.

“— but even I know that’s impossible. It’s a time whatchamacallit … a paradox!”

Read more science fiction from Nature Futures

She leant forward with a gleam in her eyes like I was 101 puppies, and she was in the market for a winter coat. “Yes, exactly! I need a paradox, a large one. Killing myself is the biggest event I can put into motion at such short notice.”

I struggled against the plastic straps that bound my hands behind my kitchen-table chair. “That doesn’t make any sense!”

“Sorry, I don’t have the time to explain the general theory of paradoxity or walk you through my calculations.”

“Calculations about what?” I asked — as long as I kept her talking, she wasn’t murdering me.

“About how much energy the death will release. Don’t worry — it will have been enough.”

“Energy for what?”

She let out an exasperated sigh. “Let me make it simple: what’s the biggest paradox you’ve heard of?”

“I don’t know — everything I say is a lie? ”

“No, that just means you don’t understand set theory. The greatest one is existence itself: why is there something instead of nothing ? It gave rise to everything, and — together with other, smaller paradoxes — keeps everything going.

“Uh huh,” I said, humouring my future self.

“But those bastards from the CCCCCC — the Chronological Continuum Consistency Coordinated Consortium Confederacy — are obsessed with timescape integrity . They’ve pushed my team back everywhen, undoing our efforts to make the timeline a better place to live in. They will even make sure World War Three — which we’d managed to avoid, you’re welcome — will begin right on time next Tuesday. I need to finish them once and for all. They’re out of control. They’ll go too far back; undo the Paradox of Life itself —”

“Life’s a paradox?”

“Duh!” — I hadn’t realized how obnoxious it is when I do that — “Why else would dumb, entropic matter organize itself into something that can laugh, love and fart?”

I looked around and saw an old family picture. “Why kill me? Wouldn’t killing somebody like … not mum or dad, um … would grandma Georgina work? We never liked her.”

“No, we didn’t. Remember the haircut incident in third grade?” She chuckled softly. “But no, sorry, it must be me, or it won’t have enough juice. A tight timeloop like this should release ten-to-the-twelfth-power chronojoules. The CCCCCC bastards will never see it coming!”

I grasped for something, anything to distract her. “Aren’t you supposed to be older? Why do you look better than me?”

She looked down at her body. “It’s a back-echo of the energy release. It rearranges nearby systems into their optimal state. And this,” she waved at herself, “is more optimal than, well, that.” She pointed at me.

“Thanks so much for taking the time to insult me before killing me.”

“No problem.” She looked at some glowing numbers on her wrist. “This will have been fun but time has run out of time — we have to do this now.”

She pulled out a knife and slipped behind me.

“Stop!” I said, but she didn’t. I felt something shift and fell forward. There was a flash of something much brighter than ordinary light could ever be.

My hands weren’t tied behind me any more. I leapt up, trying to remember the three weeks of taekwondo I’d taken back in high school — and hoping she didn’t. I turned and saw a hotter version of myself lying on the floor with a gash on the side of her throat. Blood was spreading out on the white carpet my ex-boyfriend had picked out. Good, I never liked it, or him — wait, why was I still breathing?

I looked down — my body had changed. I looked like her now. I felt the energy and knowledge move through me. I knew what I had to do — fight those bastards from the CCCCCC and win.

There was just one thing I didn’t understand. I knelt beside her. “This doesn’t make any sense. I thought you had to kill me?”

She looked up with a small, weak smile. I leant in to hear her say, “If it made sense, it wouldn’t be a paradox, would it?”

The story behind the story

Rodrigo Culagovski reveals the inspiration behind The real time-travel paradox was the friends we made along the way .

My offspring and I love to watch superhero team TV series. They usually feature some — or a lot — of time travel, and are full of plot holes and paradoxes, to the point where we joke that time-travel paradoxes are their real super power.

I’m also a member of Codex, an SFF writers community. We hold flash-fiction contests twice a year. Last year, one of the prompts was “Road trip! Where are you going and who are you bringing with?” I didn’t use it as is, but it got me thinking of my favourite snowclone, “The Real X Was the Friends We Made Along the Way”.

This story is the love child of these two ideas.

doi: https://doi.org/10.1038/d41586-024-00897-w

Related Articles

The warfighter

Futures 20 MAR 24

The neuroscientist formerly known as Prince’s audio engineer

Career Feature 14 MAR 24

Plutopalooza

Futures 13 MAR 24

Tenure-track Assistant Professor in Ecological and Evolutionary Modeling

Tenure-track Assistant Professor in Ecosystem Ecology linked to IceLab’s Center for modeling adaptive mechanisms in living systems under stress

Umeå, Sweden

Umeå University

Faculty Positions in Westlake University

Founded in 2018, Westlake University is a new type of non-profit research-oriented university in Hangzhou, China, supported by public a...

Hangzhou, Zhejiang, China

Westlake University

Postdoctoral Fellowships-Metabolic control of cell growth and senescence

Postdoctoral positions in the team Cell growth control by nutrients at Inst. Necker, Université Paris Cité, Inserm, Paris, France.

Paris, Ile-de-France (FR)

Inserm DR IDF Paris Centre Nord

Zhejiang Provincial Hospital of Chinese Medicine on Open Recruitment of Medical Talents and Postdocs

Director of Clinical Department, Professor, Researcher, Post-doctor

The First Affiliated Hospital of Zhejiang Chinese Medical University

Sir Run Run Shaw Hospital, School of Medicine, Zhejiang University, Warmly Welcomes Talents Abroad

“Qiushi” Distinguished Scholar, Zhejiang University, including Professor and Physician

No. 3, Qingchun East Road, Hangzhou, Zhejiang (CN)

Sir Run Run Shaw Hospital Affiliated with Zhejiang University School of Medicine

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Quick links

• Explore articles by subject
• Guide to authors
• Editorial policies

Is time travel even possible? An astrophysicist explains the science behind the science fiction

Assistant Professor of Astronomy and Astrophysics, University of Maryland, Baltimore County

Disclosure statement

Adi Foord does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

University of Maryland, Baltimore County provides funding as a member of The Conversation US.

View all partners

Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to [email protected] .

Will it ever be possible for time travel to occur? – Alana C., age 12, Queens, New York

Have you ever dreamed of traveling through time, like characters do in science fiction movies? For centuries, the concept of time travel has captivated people’s imaginations. Time travel is the concept of moving between different points in time, just like you move between different places. In movies, you might have seen characters using special machines, magical devices or even hopping into a futuristic car to travel backward or forward in time.

But is this just a fun idea for movies, or could it really happen?

The question of whether time is reversible remains one of the biggest unresolved questions in science. If the universe follows the laws of thermodynamics , it may not be possible. The second law of thermodynamics states that things in the universe can either remain the same or become more disordered over time.

It’s a bit like saying you can’t unscramble eggs once they’ve been cooked. According to this law, the universe can never go back exactly to how it was before. Time can only go forward, like a one-way street.

Time is relative

However, physicist Albert Einstein’s theory of special relativity suggests that time passes at different rates for different people. Someone speeding along on a spaceship moving close to the speed of light – 671 million miles per hour! – will experience time slower than a person on Earth.

People have yet to build spaceships that can move at speeds anywhere near as fast as light, but astronauts who visit the International Space Station orbit around the Earth at speeds close to 17,500 mph. Astronaut Scott Kelly has spent 520 days at the International Space Station, and as a result has aged a little more slowly than his twin brother – and fellow astronaut – Mark Kelly. Scott used to be 6 minutes younger than his twin brother. Now, because Scott was traveling so much faster than Mark and for so many days, he is 6 minutes and 5 milliseconds younger .

Some scientists are exploring other ideas that could theoretically allow time travel. One concept involves wormholes , or hypothetical tunnels in space that could create shortcuts for journeys across the universe. If someone could build a wormhole and then figure out a way to move one end at close to the speed of light – like the hypothetical spaceship mentioned above – the moving end would age more slowly than the stationary end. Someone who entered the moving end and exited the wormhole through the stationary end would come out in their past.

However, wormholes remain theoretical: Scientists have yet to spot one. It also looks like it would be incredibly challenging to send humans through a wormhole space tunnel.

Paradoxes and failed dinner parties

There are also paradoxes associated with time travel. The famous “ grandfather paradox ” is a hypothetical problem that could arise if someone traveled back in time and accidentally prevented their grandparents from meeting. This would create a paradox where you were never born, which raises the question: How could you have traveled back in time in the first place? It’s a mind-boggling puzzle that adds to the mystery of time travel.

Famously, physicist Stephen Hawking tested the possibility of time travel by throwing a dinner party where invitations noting the date, time and coordinates were not sent out until after it had happened. His hope was that his invitation would be read by someone living in the future, who had capabilities to travel back in time. But no one showed up.

As he pointed out : “The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future.”

Telescopes are time machines

Interestingly, astrophysicists armed with powerful telescopes possess a unique form of time travel. As they peer into the vast expanse of the cosmos, they gaze into the past universe. Light from all galaxies and stars takes time to travel, and these beams of light carry information from the distant past. When astrophysicists observe a star or a galaxy through a telescope, they are not seeing it as it is in the present, but as it existed when the light began its journey to Earth millions to billions of years ago.

NASA’s newest space telescope, the James Webb Space Telescope , is peering at galaxies that were formed at the very beginning of the Big Bang, about 13.7 billion years ago.

While we aren’t likely to have time machines like the ones in movies anytime soon, scientists are actively researching and exploring new ideas. But for now, we’ll have to enjoy the idea of time travel in our favorite books, movies and dreams.

Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to [email protected] . Please tell us your name, age and the city where you live.

And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.

• Time travel
• Special Relativity
• Thermodynamics
• Stephen Hawking
• Curious Kids
• Curious Kids US
• Time travel paradox

Biocloud Project Manager - Australian Biocommons

Director, Defence and Security

Opportunities with the new CIEHF

School of Social Sciences – Public Policy and International Relations opportunities

Deputy Editor - Technology

• Table of Contents
• Random Entry
• Chronological
• Editorial Information
• About the SEP
• Editorial Board
• How to Cite the SEP
• Special Characters
• Advanced Tools
• Support the SEP
• PDFs for SEP Friends
• Make a Donation
• SEPIA for Libraries
• Entry Contents

Bibliography

Academic tools.

• Friends PDF Preview
• Author and Citation Info
• Back to Top

Time Travel

There is an extensive literature on time travel in both philosophy and physics. Part of the great interest of the topic stems from the fact that reasons have been given both for thinking that time travel is physically possible—and for thinking that it is logically impossible! This entry deals primarily with philosophical issues; issues related to the physics of time travel are covered in the separate entries on time travel and modern physics and time machines . We begin with the definitional question: what is time travel? We then turn to the major objection to the possibility of backwards time travel: the Grandfather paradox. Next, issues concerning causation are discussed—and then, issues in the metaphysics of time and change. We end with a discussion of the question why, if backwards time travel will ever occur, we have not been visited by time travellers from the future.

1.1 Time Discrepancy

1.2 changing the past, 2.1 can and cannot, 2.2 improbable coincidences, 2.3 inexplicable occurrences, 3.1 backwards causation, 3.2 causal loops, 4.1 time travel and time, 4.2 time travel and change, 5. where are the time travellers, other internet resources, related entries, 1. what is time travel.

There is a number of rather different scenarios which would seem, intuitively, to count as ‘time travel’—and a number of scenarios which, while sharing certain features with some of the time travel cases, seem nevertheless not to count as genuine time travel: [ 1 ]

Time travel Doctor . Doctor Who steps into a machine in 2024. Observers outside the machine see it disappear. Inside the machine, time seems to Doctor Who to pass for ten minutes. Observers in 1984 (or 3072) see the machine appear out of nowhere. Doctor Who steps out. [ 2 ] Leap . The time traveller takes hold of a special device (or steps into a machine) and suddenly disappears; she appears at an earlier (or later) time. Unlike in Doctor , the time traveller experiences no lapse of time between her departure and arrival: from her point of view, she instantaneously appears at the destination time. [ 3 ] Putnam . Oscar Smith steps into a machine in 2024. From his point of view, things proceed much as in Doctor : time seems to Oscar Smith to pass for a while; then he steps out in 1984. For observers outside the machine, things proceed differently. Observers of Oscar’s arrival in the past see a time machine suddenly appear out of nowhere and immediately divide into two copies of itself: Oscar Smith steps out of one; and (through the window) they see inside the other something that looks just like what they would see if a film of Oscar Smith were played backwards (his hair gets shorter; food comes out of his mouth and goes back into his lunch box in a pristine, uneaten state; etc.). Observers of Oscar’s departure from the future do not simply see his time machine disappear after he gets into it: they see it collide with the apparently backwards-running machine just described, in such a way that both are simultaneously annihilated. [ 4 ] Gödel . The time traveller steps into an ordinary rocket ship (not a special time machine) and flies off on a certain course. At no point does she disappear (as in Leap ) or ‘turn back in time’ (as in Putnam )—yet thanks to the overall structure of spacetime (as conceived in the General Theory of Relativity), the traveller arrives at a point in the past (or future) of her departure. (Compare the way in which someone can travel continuously westwards, and arrive to the east of her departure point, thanks to the overall curved structure of the surface of the earth.) [ 5 ] Einstein . The time traveller steps into an ordinary rocket ship and flies off at high speed on a round trip. When he returns to Earth, thanks to certain effects predicted by the Special Theory of Relativity, only a very small amount of time has elapsed for him—he has aged only a few months—while a great deal of time has passed on Earth: it is now hundreds of years in the future of his time of departure. [ 6 ] Not time travel Sleep . One is very tired, and falls into a deep sleep. When one awakes twelve hours later, it seems from one’s own point of view that hardly any time has passed. Coma . One is in a coma for a number of years and then awakes, at which point it seems from one’s own point of view that hardly any time has passed. Cryogenics . One is cryogenically frozen for hundreds of years. Upon being woken, it seems from one’s own point of view that hardly any time has passed. Virtual . One enters a highly realistic, interactive virtual reality simulator in which some past era has been recreated down to the finest detail. Crystal . One looks into a crystal ball and sees what happened at some past time, or will happen at some future time. (Imagine that the crystal ball really works—like a closed-circuit security monitor, except that the vision genuinely comes from some past or future time. Even so, the person looking at the crystal ball is not thereby a time traveller.) Waiting . One enters one’s closet and stays there for seven hours. When one emerges, one has ‘arrived’ seven hours in the future of one’s ‘departure’. Dateline . One departs at 8pm on Monday, flies for fourteen hours, and arrives at 10pm on Monday.

A satisfactory definition of time travel would, at least, need to classify the cases in the right way. There might be some surprises—perhaps, on the best definition of ‘time travel’, Cryogenics turns out to be time travel after all—but it should certainly be the case, for example, that Gödel counts as time travel and that Sleep and Waiting do not. [ 7 ]

In fact there is no entirely satisfactory definition of ‘time travel’ in the literature. The most popular definition is the one given by Lewis (1976, 145–6):

What is time travel? Inevitably, it involves a discrepancy between time and time. Any traveller departs and then arrives at his destination; the time elapsed from departure to arrival…is the duration of the journey. But if he is a time traveller, the separation in time between departure and arrival does not equal the duration of his journey.…How can it be that the same two events, his departure and his arrival, are separated by two unequal amounts of time?…I reply by distinguishing time itself, external time as I shall also call it, from the personal time of a particular time traveller: roughly, that which is measured by his wristwatch. His journey takes an hour of his personal time, let us say…But the arrival is more than an hour after the departure in external time, if he travels toward the future; or the arrival is before the departure in external time…if he travels toward the past.

This correctly excludes Waiting —where the length of the ‘journey’ precisely matches the separation between ‘arrival’ and ‘departure’—and Crystal , where there is no journey at all—and it includes Doctor . It has trouble with Gödel , however—because when the overall structure of spacetime is as twisted as it is in the sort of case Gödel imagined, the notion of external time (“time itself”) loses its grip.

Another definition of time travel that one sometimes encounters in the literature (Arntzenius, 2006, 602) (Smeenk and Wüthrich, 2011, 5, 26) equates time travel with the existence of CTC’s: closed timelike curves. A curve in this context is a line in spacetime; it is timelike if it could represent the career of a material object; and it is closed if it returns to its starting point (i.e. in spacetime—not merely in space). This now includes Gödel —but it excludes Einstein .

The lack of an adequate definition of ‘time travel’ does not matter for our purposes here. [ 8 ] It suffices that we have clear cases of (what would count as) time travel—and that these cases give rise to all the problems that we shall wish to discuss.

Some authors (in philosophy, physics and science fiction) consider ‘time travel’ scenarios in which there are two temporal dimensions (e.g. Meiland (1974)), and others consider scenarios in which there are multiple ‘parallel’ universes—each one with its own four-dimensional spacetime (e.g. Deutsch and Lockwood (1994)). There is a question whether travelling to another version of 2001 (i.e. not the very same version one experienced in the past)—a version at a different point on the second time dimension, or in a different parallel universe—is really time travel, or whether it is more akin to Virtual . In any case, this kind of scenario does not give rise to many of the problems thrown up by the idea of travelling to the very same past one experienced in one’s younger days. It is these problems that form the primary focus of the present entry, and so we shall not have much to say about other kinds of ‘time travel’ scenario in what follows.

One objection to the possibility of time travel flows directly from attempts to define it in anything like Lewis’s way. The worry is that because time travel involves “a discrepancy between time and time”, time travel scenarios are simply incoherent. The time traveller traverses thirty years in one year; she is 51 years old 21 years after her birth; she dies at the age of 100, 200 years before her birth; and so on. The objection is that these are straightforward contradictions: the basic description of what time travel involves is inconsistent; therefore time travel is logically impossible. [ 9 ]

There must be something wrong with this objection, because it would show Einstein to be logically impossible—whereas this sort of future-directed time travel has actually been observed (albeit on a much smaller scale—but that does not affect the present point) (Hafele and Keating, 1972b,a). The most common response to the objection is that there is no contradiction because the interval of time traversed by the time traveller and the duration of her journey are measured with respect to different frames of reference: there is thus no reason why they should coincide. A similar point applies to the discrepancy between the time elapsed since the time traveller’s birth and her age upon arrival. There is no more of a contradiction here than in the fact that Melbourne is both 800 kilometres away from Sydney—along the main highway—and 1200 kilometres away—along the coast road. [ 10 ]

Before leaving the question ‘What is time travel?’ we should note the crucial distinction between changing the past and participating in (aka affecting or influencing) the past. [ 11 ] In the popular imagination, backwards time travel would allow one to change the past: to right the wrongs of history, to prevent one’s younger self doing things one later regretted, and so on. In a model with a single past, however, this idea is incoherent: the very description of the case involves a contradiction (e.g. the time traveller burns all her diaries at midnight on her fortieth birthday in 1976, and does not burn all her diaries at midnight on her fortieth birthday in 1976). It is not as if there are two versions of the past: the original one, without the time traveller present, and then a second version, with the time traveller playing a role. There is just one past—and two perspectives on it: the perspective of the younger self, and the perspective of the older time travelling self. If these perspectives are inconsistent (e.g. an event occurs in one but not the other) then the time travel scenario is incoherent.

This means that time travellers can do less than we might have hoped: they cannot right the wrongs of history; they cannot even stir a speck of dust on a certain day in the past if, on that day, the speck was in fact unmoved. But this does not mean that time travellers must be entirely powerless in the past: while they cannot do anything that did not actually happen, they can (in principle) do anything that did happen. Time travellers cannot change the past: they cannot make it different from the way it was—but they can participate in it: they can be amongst the people who did make the past the way it was. [ 12 ]

What about models involving two temporal dimensions, or parallel universes—do they allow for coherent scenarios in which the past is changed? [ 13 ] There is certainly no contradiction in saying that the time traveller burns all her diaries at midnight on her fortieth birthday in 1976 in universe 1 (or at hypertime A ), and does not burn all her diaries at midnight on her fortieth birthday in 1976 in universe 2 (or at hypertime B ). The question is whether this kind of story involves changing the past in the sense originally envisaged: righting the wrongs of history, preventing subsequently regretted actions, and so on. Goddu (2003) and van Inwagen (2010) argue that it does (in the context of particular hypertime models), while Smith (1997, 365–6; 2015) argues that it does not: that it involves avoiding the past—leaving it untouched while travelling to a different version of the past in which things proceed differently.

2. The Grandfather Paradox

The most important objection to the logical possibility of backwards time travel is the so-called Grandfather paradox. This paradox has actually convinced many people that backwards time travel is impossible:

The dead giveaway that true time-travel is flatly impossible arises from the well-known “paradoxes” it entails. The classic example is “What if you go back into the past and kill your grandfather when he was still a little boy?”…So complex and hopeless are the paradoxes…that the easiest way out of the irrational chaos that results is to suppose that true time-travel is, and forever will be, impossible. (Asimov 1995 [2003, 276–7]) travel into one’s past…would seem to give rise to all sorts of logical problems, if you were able to change history. For example, what would happen if you killed your parents before you were born. It might be that one could avoid such paradoxes by some modification of the concept of free will. But this will not be necessary if what I call the chronology protection conjecture is correct: The laws of physics prevent closed timelike curves from appearing . (Hawking, 1992, 604) [ 14 ]

The paradox comes in different forms. Here’s one version:

If time travel was logically possible then the time traveller could return to the past and in a suicidal rage destroy his time machine before it was completed and murder his younger self. But if this was so a necessary condition for the time trip to have occurred at all is removed, and we should then conclude that the time trip did not occur. Hence if the time trip did occur, then it did not occur. Hence it did not occur, and it is necessary that it did not occur. To reply, as it is standardly done, that our time traveller cannot change the past in this way, is a petitio principii . Why is it that the time traveller is constrained in this way? What mysterious force stills his sudden suicidal rage? (Smith, 1985, 58)

The idea is that backwards time travel is impossible because if it occurred, time travellers would attempt to do things such as kill their younger selves (or their grandfathers etc.). We know that doing these things—indeed, changing the past in any way—is impossible. But were there time travel, there would then be nothing left to stop these things happening. If we let things get to the stage where the time traveller is facing Grandfather with a loaded weapon, then there is nothing left to prevent the impossible from occurring. So we must draw the line earlier: it must be impossible for someone to get into this situation at all; that is, backwards time travel must be impossible.

In order to defend the possibility of time travel in the face of this argument we need to show that time travel is not a sure route to doing the impossible. So, given that a time traveller has gone to the past and is facing Grandfather, what could stop her killing Grandfather? Some science fiction authors resort to the idea of chaperones or time guardians who prevent time travellers from changing the past—or to mysterious forces of logic. But it is hard to take these ideas seriously—and more importantly, it is hard to make them work in detail when we remember that changing the past is impossible. (The chaperone is acting to ensure that the past remains as it was—but the only reason it ever was that way is because of his very actions.) [ 15 ] Fortunately there is a better response—also to be found in the science fiction literature, and brought to the attention of philosophers by Lewis (1976). What would stop the time traveller doing the impossible? She would fail “for some commonplace reason”, as Lewis (1976, 150) puts it. Her gun might jam, a noise might distract her, she might slip on a banana peel, etc. Nothing more than such ordinary occurrences is required to stop the time traveller killing Grandfather. Hence backwards time travel does not entail the occurrence of impossible events—and so the above objection is defused.

A problem remains. Suppose Tim, a time-traveller, is facing his grandfather with a loaded gun. Can Tim kill Grandfather? On the one hand, yes he can. He is an excellent shot; there is no chaperone to stop him; the laws of logic will not magically stay his hand; he hates Grandfather and will not hesitate to pull the trigger; etc. On the other hand, no he can’t. To kill Grandfather would be to change the past, and no-one can do that (not to mention the fact that if Grandfather died, then Tim would not have been born). So we have a contradiction: Tim can kill Grandfather and Tim cannot kill Grandfather. Time travel thus leads to a contradiction: so it is impossible.

Note the difference between this version of the Grandfather paradox and the version considered above. In the earlier version, the contradiction happens if Tim kills Grandfather. The solution was to say that Tim can go into the past without killing Grandfather—hence time travel does not entail a contradiction. In the new version, the contradiction happens as soon as Tim gets to the past. Of course Tim does not kill Grandfather—but we still have a contradiction anyway: for he both can do it, and cannot do it. As Lewis puts it:

Could a time traveler change the past? It seems not: the events of a past moment could no more change than numbers could. Yet it seems that he would be as able as anyone to do things that would change the past if he did them. If a time traveler visiting the past both could and couldn’t do something that would change it, then there cannot possibly be such a time traveler. (Lewis, 1976, 149)

Lewis’s own solution to this problem has been widely accepted. [ 16 ] It turns on the idea that to say that something can happen is to say that its occurrence is compossible with certain facts, where context determines (more or less) which facts are the relevant ones. Tim’s killing Grandfather in 1921 is compossible with the facts about his weapon, training, state of mind, and so on. It is not compossible with further facts, such as the fact that Grandfather did not die in 1921. Thus ‘Tim can kill Grandfather’ is true in one sense (relative to one set of facts) and false in another sense (relative to another set of facts)—but there is no single sense in which it is both true and false. So there is no contradiction here—merely an equivocation.

Another response is that of Vihvelin (1996), who argues that there is no contradiction here because ‘Tim can kill Grandfather’ is simply false (i.e. contra Lewis, there is no legitimate sense in which it is true). According to Vihvelin, for ‘Tim can kill Grandfather’ to be true, there must be at least some occasions on which ‘If Tim had tried to kill Grandfather, he would or at least might have succeeded’ is true—but, Vihvelin argues, at any world remotely like ours, the latter counterfactual is always false. [ 17 ]

Return to the original version of the Grandfather paradox and Lewis’s ‘commonplace reasons’ response to it. This response engenders a new objection—due to Horwich (1987)—not to the possibility but to the probability of backwards time travel.

Think about correlated events in general. Whenever we see two things frequently occurring together, this is because one of them causes the other, or some third thing causes both. Horwich calls this the Principle of V-Correlation:

if events of type A and B are associated with one another, then either there is always a chain of events between them…or else we find an earlier event of type C that links up with A and B by two such chains of events. What we do not see is…an inverse fork—in which A and B are connected only with a characteristic subsequent event, but no preceding one. (Horwich, 1987, 97–8)

For example, suppose that two students turn up to class wearing the same outfits. That could just be a coincidence (i.e. there is no common cause, and no direct causal link between the two events). If it happens every week for the whole semester, it is possible that it is a coincidence, but this is extremely unlikely . Normally, we see this sort of extensive correlation only if either there is a common cause (e.g. both students have product endorsement deals with the same clothing company, or both slavishly copy the same influencer) or a direct causal link (e.g. one student is copying the other).

Now consider the time traveller setting off to kill her younger self. As discussed, no contradiction need ensue—this is prevented not by chaperones or mysterious forces, but by a run of ordinary occurrences in which the trigger falls off the time traveller’s gun, a gust of wind pushes her bullet off course, she slips on a banana peel, and so on. But now consider this run of ordinary occurrences. Whenever the time traveller contemplates auto-infanticide, someone nearby will drop a banana peel ready for her to slip on, or a bird will begin to fly so that it will be in the path of the time traveller’s bullet by the time she fires, and so on. In general, there will be a correlation between auto-infanticide attempts and foiling occurrences such as the presence of banana peels—and this correlation will be of the type that does not involve a direct causal connection between the correlated events or a common cause of both. But extensive correlations of this sort are, as we saw, extremely rare—so backwards time travel will happen about as often as you will see two people wear the same outfits to class every day of semester, without there being any causal connection between what one wears and what the other wears.

We can set out Horwich’s argument this way:

• If time travel were ever to occur, we should see extensive uncaused correlations.
• It is extremely unlikely that we should ever see extensive uncaused correlations.
• Therefore time travel is extremely unlikely to occur.

The conclusion is not that time travel is impossible, but that we should treat it the way we treat the possibility of, say, tossing a fair coin and getting heads one thousand times in a row. As Price (1996, 278 n.7) puts it—in the context of endorsing Horwich’s conclusion: “the hypothesis of time travel can be made to imply propositions of arbitrarily low probability. This is not a classical reductio, but it is as close as science ever gets.”

Smith (1997) attacks both premisses of Horwich’s argument. Against the first premise, he argues that backwards time travel, in itself, does not entail extensive uncaused correlations. Rather, when we look more closely, we see that time travel scenarios involving extensive uncaused correlations always build in prior coincidences which are themselves highly unlikely. Against the second premise, he argues that, from the fact that we have never seen extensive uncaused correlations, it does not follow that we never shall. This is not inductive scepticism: let us assume (contra the inductive sceptic) that in the absence of any specific reason for thinking things should be different in the future, we are entitled to assume they will continue being the same; still we cannot dismiss a specific reason for thinking the future will be a certain way simply on the basis that things have never been that way in the past. You might reassure an anxious friend that the sun will certainly rise tomorrow because it always has in the past—but you cannot similarly refute an astronomer who claims to have discovered a specific reason for thinking that the earth will stop rotating overnight.

Sider (2002, 119–20) endorses Smith’s second objection. Dowe (2003) criticises Smith’s first objection, but agrees with the second, concluding overall that time travel has not been shown to be improbable. Ismael (2003) reaches a similar conclusion. Goddu (2007) criticises Smith’s first objection to Horwich. Further contributions to the debate include Arntzenius (2006), Smeenk and Wüthrich (2011, §2.2) and Elliott (2018). For other arguments to the same conclusion as Horwich’s—that time travel is improbable—see Ney (2000) and Effingham (2020).

Return again to the original version of the Grandfather paradox and Lewis’s ‘commonplace reasons’ response to it. This response engenders a further objection. The autoinfanticidal time traveller is attempting to do something impossible (render herself permanently dead from an age younger than her age at the time of the attempts). Suppose we accept that she will not succeed and that what will stop her is a succession of commonplace occurrences. The previous objection was that such a succession is improbable . The new objection is that the exclusion of the time traveler from successfully committing auto-infanticide is mysteriously inexplicable . The worry is as follows. Each particular event that foils the time traveller is explicable in a perfectly ordinary way; but the inevitable combination of these events amounts to a ring-fencing of the forbidden zone of autoinfanticide—and this ring-fencing is mystifying. It’s like a grand conspiracy to stop the time traveler from doing what she wants to do—and yet there are no conspirators: no time lords, no magical forces of logic. This is profoundly perplexing. Riggs (1997, 52) writes: “Lewis’s account may do for a once only attempt, but is untenable as a general explanation of Tim’s continual lack of success if he keeps on trying.” Ismael (2003, 308) writes: “Considered individually, there will be nothing anomalous in the explanations…It is almost irresistible to suppose, however, that there is something anomalous in the cases considered collectively, i.e., in our unfailing lack of success.” See also Gorovitz (1964, 366–7), Horwich (1987, 119–21) and Carroll (2010, 86).

There have been two different kinds of defense of time travel against the objection that it involves mysteriously inexplicable occurrences. Baron and Colyvan (2016, 70) agree with the objectors that a purely causal explanation of failure—e.g. Tim fails to kill Grandfather because first he slips on a banana peel, then his gun jams, and so on—is insufficient. However they argue that, in addition, Lewis offers a non-causal—a logical —explanation of failure: “What explains Tim’s failure to kill his grandfather, then, is something about logic; specifically: Tim fails to kill his grandfather because the law of non-contradiction holds.” Smith (2017) argues that the appearance of inexplicability is illusory. There are no scenarios satisfying the description ‘a time traveller commits autoinfanticide’ (or changes the past in any other way) because the description is self-contradictory (e.g. it involves the time traveller permanently dying at 20 and also being alive at 40). So whatever happens it will not be ‘that’. There is literally no way for the time traveller not to fail. Hence there is no need for—or even possibility of—a substantive explanation of why failure invariably occurs, and such failure is not perplexing.

3. Causation

Backwards time travel scenarios give rise to interesting issues concerning causation. In this section we examine two such issues.

Earlier we distinguished changing the past and affecting the past, and argued that while the former is impossible, backwards time travel need involve only the latter. Affecting the past would be an example of backwards causation (i.e. causation where the effect precedes its cause)—and it has been argued that this too is impossible, or at least problematic. [ 18 ] The classic argument against backwards causation is the bilking argument . [ 19 ] Faced with the claim that some event A causes an earlier event B , the proponent of the bilking objection recommends an attempt to decorrelate A and B —that is, to bring about A in cases in which B has not occurred, and to prevent A in cases in which B has occurred. If the attempt is successful, then B often occurs despite the subsequent nonoccurrence of A , and A often occurs without B occurring, and so A cannot be the cause of B . If, on the other hand, the attempt is unsuccessful—if, that is, A cannot be prevented when B has occurred, nor brought about when B has not occurred—then, it is argued, it must be B that is the cause of A , rather than vice versa.

The bilking procedure requires repeated manipulation of event A . Thus, it cannot get under way in cases in which A is either unrepeatable or unmanipulable. Furthermore, the procedure requires us to know whether or not B has occurred, prior to manipulating A —and thus, it cannot get under way in cases in which it cannot be known whether or not B has occurred until after the occurrence or nonoccurrence of A (Dummett, 1964). These three loopholes allow room for many claims of backwards causation that cannot be touched by the bilking argument, because the bilking procedure cannot be performed at all. But what about those cases in which it can be performed? If the procedure succeeds—that is, A and B are decorrelated—then the claim that A causes B is refuted, or at least weakened (depending upon the details of the case). But if the bilking attempt fails, it does not follow that it must be B that is the cause of A , rather than vice versa. Depending upon the situation, that B causes A might become a viable alternative to the hypothesis that A causes B —but there is no reason to think that this alternative must always be the superior one. For example, suppose that I see a photo of you in a paper dated well before your birth, accompanied by a report of your arrival from the future. I now try to bilk your upcoming time trip—but I slip on a banana peel while rushing to push you away from your time machine, my time travel horror stories only inspire you further, and so on. Or again, suppose that I know that you were not in Sydney yesterday. I now try to get you to go there in your time machine—but first I am struck by lightning, then I fall down a manhole, and so on. What does all this prove? Surely not that your arrival in the past causes your departure from the future. Depending upon the details of the case, it seems that we might well be entitled to describe it as involving backwards time travel and backwards causation. At least, if we are not so entitled, this must be because of other facts about the case: it would not follow simply from the repeated coincidental failures of my bilking attempts.

Backwards time travel would apparently allow for the possibility of causal loops, in which things come from nowhere. The things in question might be objects—imagine a time traveller who steals a time machine from the local museum in order to make his time trip and then donates the time machine to the same museum at the end of the trip (i.e. in the past). In this case the machine itself is never built by anyone—it simply exists. The things in question might be information—imagine a time traveller who explains the theory behind time travel to her younger self: theory that she herself knows only because it was explained to her in her youth by her time travelling older self. The things in question might be actions. Imagine a time traveller who visits his younger self. When he encounters his younger self, he suddenly has a vivid memory of being punched on the nose by a strange visitor. He realises that this is that very encounter—and resignedly proceeds to punch his younger self. Why did he do it? Because he knew that it would happen and so felt that he had to do it—but he only knew it would happen because he in fact did it. [ 20 ]

One might think that causal loops are impossible—and hence that insofar as backwards time travel entails such loops, it too is impossible. [ 21 ] There are two issues to consider here. First, does backwards time travel entail causal loops? Lewis (1976, 148) raises the question whether there must be causal loops whenever there is backwards causation; in response to the question, he says simply “I am not sure.” Mellor (1998, 131) appears to claim a positive answer to the question. [ 22 ] Hanley (2004, 130) defends a negative answer by telling a time travel story in which there is backwards time travel and backwards causation, but no causal loops. [ 23 ] Monton (2009) criticises Hanley’s counterexample, but also defends a negative answer via different counterexamples. Effingham (2020) too argues for a negative answer.

Second, are causal loops impossible, or in some other way objectionable? One objection is that causal loops are inexplicable . There have been two main kinds of response to this objection. One is to agree but deny that this is a problem. Lewis (1976, 149) accepts that a loop (as a whole) would be inexplicable—but thinks that this inexplicability (like that of the Big Bang or the decay of a tritium atom) is merely strange, not impossible. In a similar vein, Meyer (2012, 263) argues that if someone asked for an explanation of a loop (as a whole), “the blame would fall on the person asking the question, not on our inability to answer it.” The second kind of response (Hanley, 2004, §5) is to deny that (all) causal loops are inexplicable. A second objection to causal loops, due to Mellor (1998, ch.12), is that in such loops the chances of events would fail to be related to their frequencies in accordance with the law of large numbers. Berkovitz (2001) and Dowe (2001) both argue that Mellor’s objection fails to establish the impossibility of causal loops. [ 24 ] Effingham (2020) considers—and rebuts—some additional objections to the possibility of causal loops.

4. Time and Change

Gödel (1949a [1990a])—in which Gödel presents models of Einstein’s General Theory of Relativity in which there exist CTC’s—can well be regarded as initiating the modern academic literature on time travel, in both philosophy and physics. In a companion paper, Gödel discusses the significance of his results for more general issues in the philosophy of time (Gödel 1949b [1990b]). For the succeeding half century, the time travel literature focussed predominantly on objections to the possibility (or probability) of time travel. More recently, however, there has been renewed interest in the connections between time travel and more general issues in the metaphysics of time and change. We examine some of these in the present section. [ 25 ]

The first thing that we need to do is set up the various metaphysical positions whose relationships with time travel will then be discussed. Consider two metaphysical questions:

• Are the past, present and future equally real?
• Is there an objective flow or passage of time, and an objective now?

We can label some views on the first question as follows. Eternalism is the view that past and future times, objects and events are just as real as the present time and present events and objects. Nowism is the view that only the present time and present events and objects exist. Now-and-then-ism is the view that the past and present exist but the future does not. We can also label some views on the second question. The A-theory answers in the affirmative: the flow of time and division of events into past (before now), present (now) and future (after now) are objective features of reality (as opposed to mere features of our experience). Furthermore, they are linked: the objective flow of time arises from the movement, through time, of the objective now (from the past towards the future). The B-theory answers in the negative: while we certainly experience now as special, and time as flowing, the B-theory denies that what is going on here is that we are detecting objective features of reality in a way that corresponds transparently to how those features are in themselves. The flow of time and the now are not objective features of reality; they are merely features of our experience. By combining answers to our first and second questions we arrive at positions on the metaphysics of time such as: [ 26 ]

• the block universe view: eternalism + B-theory
• the moving spotlight view: eternalism + A-theory
• the presentist view: nowism + A-theory
• the growing block view: now-and-then-ism + A-theory.

So much for positions on time itself. Now for some views on temporal objects: objects that exist in (and, in general, change over) time. Three-dimensionalism is the view that persons, tables and other temporal objects are three-dimensional entities. On this view, what you see in the mirror is a whole person. [ 27 ] Tomorrow, when you look again, you will see the whole person again. On this view, persons and other temporal objects are wholly present at every time at which they exist. Four-dimensionalism is the view that persons, tables and other temporal objects are four-dimensional entities, extending through three dimensions of space and one dimension of time. On this view, what you see in the mirror is not a whole person: it is just a three-dimensional temporal part of a person. Tomorrow, when you look again, you will see a different such temporal part. Say that an object persists through time if it is around at some time and still around at a later time. Three- and four-dimensionalists agree that (some) objects persist, but they differ over how objects persist. According to three-dimensionalists, objects persist by enduring : an object persists from t 1 to t 2 by being wholly present at t 1 and t 2 and every instant in between. According to four-dimensionalists, objects persist by perduring : an object persists from t 1 to t 2 by having temporal parts at t 1 and t 2 and every instant in between. Perduring can be usefully compared with being extended in space: a road extends from Melbourne to Sydney not by being wholly located at every point in between, but by having a spatial part at every point in between.

It is natural to combine three-dimensionalism with presentism and four-dimensionalism with the block universe view—but other combinations of views are certainly possible.

Gödel (1949b [1990b]) argues from the possibility of time travel (more precisely, from the existence of solutions to the field equations of General Relativity in which there exist CTC’s) to the B-theory: that is, to the conclusion that there is no objective flow or passage of time and no objective now. Gödel begins by reviewing an argument from Special Relativity to the B-theory: because the notion of simultaneity becomes a relative one in Special Relativity, there is no room for the idea of an objective succession of “nows”. He then notes that this argument is disrupted in the context of General Relativity, because in models of the latter theory to date, the presence of matter does allow recovery of an objectively distinguished series of “nows”. Gödel then proposes a new model (Gödel 1949a [1990a]) in which no such recovery is possible. (This is the model that contains CTC’s.) Finally, he addresses the issue of how one can infer anything about the nonexistence of an objective flow of time in our universe from the existence of a merely possible universe in which there is no objectively distinguished series of “nows”. His main response is that while it would not be straightforwardly contradictory to suppose that the existence of an objective flow of time depends on the particular, contingent arrangement and motion of matter in the world, this would nevertheless be unsatisfactory. Responses to Gödel have been of two main kinds. Some have objected to the claim that there is no objective flow of time in his model universe (e.g. Savitt (2005); see also Savitt (1994)). Others have objected to the attempt to transfer conclusions about that model universe to our own universe (e.g. Earman (1995, 197–200); for a partial response to Earman see Belot (2005, §3.4)). [ 28 ]

Earlier we posed two questions:

Gödel’s argument is related to the second question. Let’s turn now to the first question. Godfrey-Smith (1980, 72) writes “The metaphysical picture which underlies time travel talk is that of the block universe [i.e. eternalism, in the terminology of the present entry], in which the world is conceived as extended in time as it is in space.” In his report on the Analysis problem to which Godfrey-Smith’s paper is a response, Harrison (1980, 67) replies that he would like an argument in support of this assertion. Here is an argument: [ 29 ]

A fundamental requirement for the possibility of time travel is the existence of the destination of the journey. That is, a journey into the past or the future would have to presuppose that the past or future were somehow real. (Grey, 1999, 56)

Dowe (2000, 442–5) responds that the destination does not have to exist at the time of departure: it only has to exist at the time of arrival—and this is quite compatible with non-eternalist views. And Keller and Nelson (2001, 338) argue that time travel is compatible with presentism:

There is four-dimensional [i.e. eternalist, in the terminology of the present entry] time-travel if the appropriate sorts of events occur at the appropriate sorts of times; events like people hopping into time-machines and disappearing, people reappearing with the right sorts of memories, and so on. But the presentist can have just the same patterns of events happening at just the same times. Or at least, it can be the case on the presentist model that the right sorts of events will happen, or did happen, or are happening, at the rights sorts of times. If it suffices for four-dimensionalist time-travel that Jennifer disappears in 2054 and appears in 1985 with the right sorts of memories, then why shouldn’t it suffice for presentist time-travel that Jennifer will disappear in 2054, and that she did appear in 1985 with the right sorts of memories?

Sider (2005) responds that there is still a problem reconciling presentism with time travel conceived in Lewis’s way: that conception of time travel requires that personal time is similar to external time—but presentists have trouble allowing this. Further contributions to the debate whether presentism—and other versions of the A-theory—are compatible with time travel include Monton (2003), Daniels (2012), Hall (2014) and Wasserman (2018) on the side of compatibility, and Miller (2005), Slater (2005), Miller (2008), Hales (2010) and Markosian (2020) on the side of incompatibility.

Leibniz’s Law says that if x = y (i.e. x and y are identical—one and the same entity) then x and y have exactly the same properties. There is a superficial conflict between this principle of logic and the fact that things change. If Bill is at one time thin and at another time not so—and yet it is the very same person both times—it looks as though the very same entity (Bill) both possesses and fails to possess the property of being thin. Three-dimensionalists and four-dimensionalists respond to this problem in different ways. According to the four-dimensionalist, what is thin is not Bill (who is a four-dimensional entity) but certain temporal parts of Bill; and what is not thin are other temporal parts of Bill. So there is no single entity that both possesses and fails to possess the property of being thin. Three-dimensionalists have several options. One is to deny that there are such properties as ‘thin’ (simpliciter): there are only temporally relativised properties such as ‘thin at time t ’. In that case, while Bill at t 1 and Bill at t 2 are the very same entity—Bill is wholly present at each time—there is no single property that this one entity both possesses and fails to possess: Bill possesses the property ‘thin at t 1 ’ and lacks the property ‘thin at t 2 ’. [ 30 ]

Now consider the case of a time traveller Ben who encounters his younger self at time t . Suppose that the younger self is thin and the older self not so. The four-dimensionalist can accommodate this scenario easily. Just as before, what we have are two different three-dimensional parts of the same four-dimensional entity, one of which possesses the property ‘thin’ and the other of which does not. The three-dimensionalist, however, faces a problem. Even if we relativise properties to times, we still get the contradiction that Ben possesses the property ‘thin at t ’ and also lacks that very same property. [ 31 ] There are several possible options for the three-dimensionalist here. One is to relativise properties not to external times but to personal times (Horwich, 1975, 434–5); another is to relativise properties to spatial locations as well as to times (or simply to spacetime points). Sider (2001, 101–6) criticises both options (and others besides), concluding that time travel is incompatible with three-dimensionalism. Markosian (2004) responds to Sider’s argument; [ 32 ] Miller (2006) also responds to Sider and argues for the compatibility of time travel and endurantism; Gilmore (2007) seeks to weaken the case against endurantism by constructing analogous arguments against perdurantism. Simon (2005) finds problems with Sider’s arguments, but presents different arguments for the same conclusion; Effingham and Robson (2007) and Benovsky (2011) also offer new arguments for this conclusion. For further discussion see Wasserman (2018) and Effingham (2020). [ 33 ]

We have seen arguments to the conclusions that time travel is impossible, improbable and inexplicable. Here’s an argument to the conclusion that backwards time travel simply will not occur. If backwards time travel is ever going to occur, we would already have seen the time travellers—but we have seen none such. [ 34 ] The argument is a weak one. [ 35 ] For a start, it is perhaps conceivable that time travellers have already visited the Earth [ 36 ] —but even granting that they have not, this is still compatible with the future actuality of backwards time travel. First, it may be that time travel is very expensive, difficult or dangerous—or for some other reason quite rare—and that by the time it is available, our present period of history is insufficiently high on the list of interesting destinations. Second, it may be—and indeed existing proposals in the physics literature have this feature—that backwards time travel works by creating a CTC that lies entirely in the future: in this case, backwards time travel becomes possible after the creation of the CTC, but travel to a time earlier than the time at which the CTC is created is not possible. [ 37 ]

• Adams, Robert Merrihew, 1997, “Thisness and time travel”, Philosophia , 25: 407–15.
• Arntzenius, Frank, 2006, “Time travel: Double your fun”, Philosophy Compass , 1: 599–616. doi:10.1111/j.1747-9991.2006.00045.x
• Asimov, Isaac, 1995 [2003], Gold: The Final Science Fiction Collection , New York: Harper Collins.
• Baron, Sam and Colyvan, Mark, 2016, “Time enough for explanation”, Journal of Philosophy , 113: 61–88.
• Belot, Gordon, 2005, “Dust, time and symmetry”, British Journal for the Philosophy of Science , 56: 255–91.
• Benovsky, Jiri, 2011, “Endurance and time travel”, Kriterion , 24: 65–72.
• Berkovitz, Joseph, 2001, “On chance in causal loops”, Mind , 110: 1–23.
• Black, Max, 1956, “Why cannot an effect precede its cause?”, Analysis , 16: 49–58.
• Brier, Bob, 1973, “Magicians, alarm clocks, and backward causation”, Southern Journal of Philosophy , 11: 359–64.
• Carlson, Erik, 2005, “A new time travel paradox resolved”, Philosophia , 33: 263–73.
• Carroll, John W., 2010, “Context, conditionals, fatalism, time travel, and freedom”, in Time and Identity , Joseph Keim Campbell, Michael O’Rourke, and Harry S. Silverstein, eds., Cambridge MA: MIT Press, 79–93.
• Craig, William L., 1997, “Adams on actualism and presentism”, Philosophia , 25: 401–5.
• Daniels, Paul R., 2012, “Back to the present: Defending presentist time travel”, Disputatio , 4: 469–84.
• Deutsch, David and Lockwood, Michael, 1994, “The quantum physics of time travel”, Scientific American , 270(3): 50–6.
• Dowe, Phil, 2000, “The case for time travel”, Philosophy , 75: 441–51.
• –––, 2001, “Causal loops and the independence of causal facts”, Philosophy of Science , 68: S89–S97.
• –––, 2003, “The coincidences of time travel”, Philosophy of Science , 70: 574–89.
• Dummett, Michael, 1964, “Bringing about the past”, Philosophical Review , 73: 338–59.
• Dwyer, Larry, 1977, “How to affect, but not change, the past”, Southern Journal of Philosophy , 15: 383–5.
• Earman, John, 1995, Bangs, Crunches, Whimpers, and Shrieks: Singularities and Acausalities in Relativistic Spacetimes , New York: Oxford University Press.
• Effingham, Nikk, 2020, Time Travel: Probability and Impossibility , Oxford: Oxford University Press.
• Effingham, Nikk and Robson, Jon, 2007, “A mereological challenge to endurantism”, Australasian Journal of Philosophy , 85: 633–40.
• Ehring, Douglas, 1997, “Personal identity and time travel”, Philosophical Studies , 52: 427–33.
• Elliott, Katrina, 2019, “How to Know That Time Travel Is Unlikely Without Knowing Why”, Pacific Philosophical Quarterly , 100: 90–113.
• Fulmer, Gilbert, 1980, “Understanding time travel”, Southwestern Journal of Philosophy , 11: 151–6.
• Gilmore, Cody, 2007, “Time travel, coinciding objects, and persistence”, in Oxford Studies in Metaphysics , Dean W. Zimmerman, ed., Oxford: Clarendon Press, vol. 3, 177–98.
• Goddu, G.C., 2003, “Time travel and changing the past (or how to kill yourself and live to tell the tale)”, Ratio , 16: 16–32.
• –––, 2007, “Banana peels and time travel”, Dialectica , 61: 559–72.
• Gödel, Kurt, 1949a [1990a], “An example of a new type of cosmological solutions of Einstein’s field equations of gravitation”, in Kurt Gödel: Collected Works (Volume II), Solomon Feferman, et al. (eds.), New York: Oxford University Press, 190–8; originally published in Reviews of Modern Physics , 21 (1949): 447–450.
• –––, 1949b [1990b], “A remark about the relationship between relativity theory and idealistic philosophy”, in Kurt Gödel: Collected Works (Volume II), Solomon Feferman, et al. (eds.), New York: Oxford University Press, 202–7; originally published in P. Schilpp (ed.), Albert Einstein: Philosopher-Scientist , La Salle: Open Court, 1949, 555–562.
• Godfrey-Smith, William, 1980, “Travelling in time”, Analysis , 40: 72–3.
• Gorovitz, Samuel, 1964, “Leaving the past alone”, Philosophical Review , 73: 360–71.
• Grey, William, 1999, “Troubles with time travel”, Philosophy , 74: 55–70.
• Hafele, J. C. and Keating, Richard E., 1972a, “Around-the-world atomic clocks: Observed relativistic time gains”, Science , 177: 168–70.
• –––, 1972b, “Around-the-world atomic clocks: Predicted relativistic time gains”, Science , 177: 166–8.
• Hales, Steven D., 2010, “No time travel for presentists”, Logos & Episteme , 1: 353–60.
• Hall, Thomas, 2014, “In Defense of the Compossibility of Presentism and Time Travel”, Logos & Episteme , 2: 141–59.
• Hanley, Richard, 2004, “No end in sight: Causal loops in philosophy, physics and fiction”, Synthese , 141: 123–52.
• Harrison, Jonathan, 1980, “Report on analysis ‘problem’ no. 18”, Analysis , 40: 65–9.
• Hawking, S.W., 1992, “Chronology protection conjecture”, Physical Review D , 46: 603–11.
• Holt, Dennis Charles, 1981, “Time travel: The time discrepancy paradox”, Philosophical Investigations , 4: 1–16.
• Horacek, David, 2005, “Time travel in indeterministic worlds”, Monist (Special Issue on Time Travel), 88: 423–36.
• Horwich, Paul, 1975, “On some alleged paradoxes of time travel”, Journal of Philosophy , 72: 432–44.
• –––, 1987, Asymmetries in Time: Problems in the Philosophy of Science , Cambridge MA: MIT Press.
• Ismael, J., 2003, “Closed causal loops and the bilking argument”, Synthese , 136: 305–20.
• Keller, Simon and Nelson, Michael, 2001, “Presentists should believe in time-travel”, Australasian Journal of Philosophy , 79: 333–45.
• Kiourti, Ira, 2008, “Killing baby Suzy”, Philosophical Studies , 139: 343–52.
• Le Poidevin, Robin, 2003, Travels in Four Dimensions: The Enigmas of Space and Time , Oxford: Oxford University Press.
• –––, 2005, “The Cheshire Cat problem and other spatial obstacles to backwards time travel”, Monist (Special Issue on Time Travel), 88: 336–52.
• Lewis, David, 1976, “The paradoxes of time travel”, American Philosophical Quarterly , 13: 145–52.
• Loss, Roberto, 2015, “How to Change the Past in One-Dimensional Time”, Pacific Philosophical Quarterly , 96: 1–11.
• Luminet, Jean-Pierre, 2011, “Time, topology, and the twin paradox”, in The Oxford Handbook of Philosophy of Time , Craig Callender (ed.), Oxford: Oxford University Press. doi:10.1093/oxfordhb/9780199298204.003.0018
• Markosian, Ned, 2004, “Two arguments from Sider’s Four-Dimensionalism ”, Philosophy and Phenomenological Research , 68: 665–73.
• Markosian, Ned, 2020, “The Dynamic Theory of Time and Time Travel to the Past”, Disputatio , 12: 137–65.
• Maudlin, Tim, 2012, Philosophy of Physics: Space and Time , Princeton: Princeton University Press.
• Meiland, Jack W., 1974, “A two-dimensional passage model of time for time travel”, Philosophical Studies , 26: 153–73.
• Mellor, D.H., 1998, Real Time II , London: Routledge.
• Meyer, Ulrich, 2012, “Explaining causal loops”, Analysis , 72: 259–64.
• Miller, Kristie, 2005, “Time travel and the open future”, Disputatio , 1: 223–32.
• –––, 2006, “Travelling in time: How to wholly exist in two places at the same time”, Canadian Journal of Philosophy , 36: 309–34.
• –––, 2008, “Backwards causation, time, and the open future”, Metaphysica , 9: 173–91.
• Monton, Bradley, 2003, “Presentists can believe in closed timelike curves”, Analysis , 63: 199–202.
• –––, 2009, “Time travel without causal loops”, Philosophical Quarterly , 59: 54–67.
• Nerlich, Graham, 1981, “Can time be finite?”, Pacific Philosophical Quarterly , 62: 227–39.
• Ney, S.E., 2000, “Are grandfathers an endangered species?”, Journal of Philosophical Research , 25: 311–21.
• Price, Huw, 1996, Time’s Arrow & Archimedes’ Point: New Directions for the Physics of Time , New York: Oxford University Press.
• Putnam, Hilary, 1975, “It ain’t necessarily so”, in Mathematics, Matter and Method , Cambridge: Cambridge University Press, vol. 1 of Philosophical Papers , 237–49.
• Reinganum, Marc R., 1986, “Is time travel impossible? A financial proof”, Journal of Portfolio Management , 13: 10–2.
• Riggs, Peter J., 1991, “A critique of Mellor’s argument against ‘backwards’ causation”, British Journal for the Philosophy of Science , 42: 75–86.
• –––, 1997, “The principal paradox of time travel”, Ratio , 10: 48–64.
• Savitt, Steven, 1994, “The replacement of time”, Australasian Journal of Philosophy , 74: 463–73.
• –––, 2005, “Time travel and becoming”, Monist (Special Issue on Time Travel), 88: 413–22.
• Sider, Theodore, 2001, Four-Dimensionalism: An Ontology of Persistence and Time , Oxford: Clarendon Press.
• –––, 2002, “Time travel, coincidences and counterfactuals”, Philosophical Studies , 110: 115–38.
• –––, 2004, “Replies to Gallois, Hirsch and Markosian”, Philosophy and Phenomenological Research , 68: 674–87.
• –––, 2005, “Traveling in A- and B- time”, Monist (Special Issue on Time Travel), 88: 329–35.
• Simon, Jonathan, 2005, “Is time travel a problem for the three-dimensionalist?”, Monist (Special Issue on Time Travel), 88: 353–61.
• Slater, Matthew H., 2005, “The necessity of time travel (on pain of indeterminacy)”, Monist (Special Issue on Time Travel), 88: 362–9.
• Smart, J.J.C., 1963, “Is time travel possible?”, Journal of Philosophy , 60: 237–41.
• Smeenk, Chris and Wüthrich, Christian, 2011, “Time travel and time machines”, in The Oxford Handbook of Philosophy of Time , Craig Callender (ed.), Oxford: Oxford University Press, online ed. doi:10.1093/oxfordhb/9780199298204.003.0021
• Smith, Joseph Wayne, 1985, “Time travel and backward causation”, Cogito , 3: 57–67.
• Smith, Nicholas J.J., 1997, “Bananas enough for time travel?”, British Journal for the Philosophy of Science , 48: 363–89.
• –––, 1998, “The problems of backward time travel”, Endeavour , 22(4): 156–8.
• –––, 2004, “Review of Robin Le Poidevin Travels in Four Dimensions: The Enigmas of Space and Time ”, Australasian Journal of Philosophy , 82: 527–30.
• –––, 2005, “Why would time travellers try to kill their younger selves?”, Monist (Special Issue on Time Travel), 88: 388–95.
• –––, 2011, “Inconsistency in the A-theory”, Philosophical Studies , 156: 231–47.
• –––, 2015, “Why time travellers (still) cannot change the past”, Revista Portuguesa de Filosofia , 71: 677–94.
• –––, 2017, “I’d do anything to change the past (but I can’t do ‘that’)”, American Philosophical Quarterly , 54: 153–68.
• van Inwagen, Peter, 2010, “Changing the past”, in Oxford Studies in Metaphysics (Volume 5), Dean W. Zimmerman (ed.), Oxford: Oxford University Press, 3–28.
• Vihvelin, Kadri, 1996, “What time travelers cannot do”, Philosophical Studies , 81: 315–30.
• Vranas, Peter B.M., 2005, “Do cry over spilt milk: Possibly you can change the past”, Monist (Special Issue on Time Travel), 88: 370–87.
• –––, 2009, “Can I kill my younger self? Time travel and the retrosuicide paradox”, Pacific Philosophical Quarterly , 90: 520–34.
• –––, 2010, “What time travelers may be able to do”, Philosophical Studies , 150: 115–21.
• Wasserman, Ryan, 2018, Paradoxes of Time Travel , Oxford: Oxford University Press.
• Williams, Donald C., 1951, “The myth of passage”, Journal of Philosophy , 48: 457–72.
• Wright, John, 2006, “Personal identity, fission and time travel”, Philosophia , 34: 129–42.
• Yourgrau, Palle, 1999, Gödel Meets Einstein: Time Travel in the Gödel Universe , Chicago: Open Court.

How to cite this entry . Preview the PDF version of this entry at the Friends of the SEP Society . Look up topics and thinkers related to this entry at the Internet Philosophy Ontology Project (InPhO). Enhanced bibliography for this entry at PhilPapers , with links to its database.

• Time Travel , entry by Joel Hunter (Truckee Meadows Community College) in the Internet Encyclopedia of Philosophy .

causation: backward | free will: divine foreknowledge and | identity: over time | location and mereology | temporal parts | time | time machines | time travel: and modern physics

Copyright © 2024 by Nicholas J.J. Smith < nicholas . smith @ sydney . edu . au >

• Accessibility

Support SEP

Mirror sites.

View this site from another server:

• Info about mirror sites

The Stanford Encyclopedia of Philosophy is copyright © 2024 by The Metaphysics Research Lab , Department of Philosophy, Stanford University

Library of Congress Catalog Data: ISSN 1095-5054

September 2, 2014

Time Travel Simulation Resolves “Grandfather Paradox”

What would happen to you if you went back in time and killed your grandfather? A model using photons reveals that quantum mechanics can solve the quandary—and even foil quantum cryptography

By Lee Billings

On June 28, 2009, the world-famous physicist Stephen Hawking threw a party at the University of Cambridge, complete with balloons, hors d'oeuvres and iced champagne. Everyone was invited but no one showed up. Hawking had expected as much, because he only sent out invitations after his party had concluded. It was, he said, "a welcome reception for future time travelers," a tongue-in-cheek experiment to reinforce his 1992 conjecture that travel into the past is effectively impossible.

But Hawking may be on the wrong side of history. Recent experiments offer tentative support for time travel's feasibility—at least from a mathematical perspective. The study cuts to the core of our understanding of the universe, and the resolution of the possibility of time travel, far from being a topic worthy only of science fiction, would have profound implications for fundamental physics as well as for practical applications such as quantum cryptography and computing.

Closed timelike curves The source of time travel speculation lies in the fact that our best physical theories seem to contain no prohibitions on traveling backward through time. The feat should be possible based on Einstein's theory of general relativity, which describes gravity as the warping of spacetime by energy and matter. An extremely powerful gravitational field, such as that produced by a spinning black hole, could in principle profoundly warp the fabric of existence so that spacetime bends back on itself. This would create a "closed timelike curve," or CTC, a loop that could be traversed to travel back in time.

On supporting science journalism

If you're enjoying this article, consider supporting our award-winning journalism by subscribing . By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.

Hawking and many other physicists find CTCs abhorrent, because any macroscopic object traveling through one would inevitably create paradoxes where cause and effect break down. In a model proposed by the theorist David Deutsch in 1991, however, the paradoxes created by CTCs could be avoided at the quantum scale because of the behavior of fundamental particles, which follow only the fuzzy rules of probability rather than strict determinism. "It's intriguing that you've got general relativity predicting these paradoxes, but then you consider them in quantum mechanical terms and the paradoxes go away," says University of Queensland physicist Tim Ralph. "It makes you wonder whether this is important in terms of formulating a theory that unifies general relativity with quantum mechanics."

Experimenting with a curve Recently Ralph and his PhD student Martin Ringbauer led a team that experimentally simulated Deutsch's model of CTCs for the very first time, testing and confirming many aspects of the two-decades-old theory. Their findings are published in Nature Communications. Much of their simulation revolved around investigating how Deutsch's model deals with the “grandfather paradox,” a hypothetical scenario in which someone uses a CTC to travel back through time to murder her own grandfather, thus preventing her own later birth. ( Scientific American is part of Nature Publishing Group.)

Deutsch's quantum solution to the grandfather paradox works something like this:

Instead of a human being traversing a CTC to kill her ancestor, imagine that a fundamental particle goes back in time to flip a switch on the particle-generating machine that created it. If the particle flips the switch, the machine emits a particle— the particle—back into the CTC; if the switch isn't flipped, the machine emits nothing. In this scenario there is no a priori deterministic certainty to the particle's emission, only a distribution of probabilities. Deutsch's insight was to postulate self-consistency in the quantum realm, to insist that any particle entering one end of a CTC must emerge at the other end with identical properties. Therefore, a particle emitted by the machine with a probability of one half would enter the CTC and come out the other end to flip the switch with a probability of one half, imbuing itself at birth with a probability of one half of going back to flip the switch. If the particle were a person, she would be born with a one-half probability of killing her grandfather, giving her grandfather a one-half probability of escaping death at her hands—good enough in probabilistic terms to close the causative loop and escape the paradox. Strange though it may be, this solution is in keeping with the known laws of quantum mechanics.

In their new simulation Ralph, Ringbauer and their colleagues studied Deutsch's model using interactions between pairs of polarized photons within a quantum system that they argue is mathematically equivalent to a single photon traversing a CTC. "We encode their polarization so that the second one acts as kind of a past incarnation of the first,” Ringbauer says. So instead of sending a person through a time loop, they created a stunt double of the person and ran him through a time-loop simulator to see if the doppelganger emerging from a CTC exactly resembled the original person as he was in that moment in the past.

By measuring the polarization states of the second photon after its interaction with the first, across multiple trials the team successfully demonstrated Deutsch's self-consistency in action. "The state we got at our output, the second photon at the simulated exit of the CTC, was the same as that of our input, the first encoded photon at the CTC entrance," Ralph says. "Of course, we're not really sending anything back in time but [the simulation] allows us to study weird evolutions normally not allowed in quantum mechanics."

Those "weird evolutions" enabled by a CTC, Ringbauer notes, would have remarkable practical applications, such as breaking quantum-based cryptography through the cloning of the quantum states of fundamental particles. "If you can clone quantum states,” he says, “you can violate the Heisenberg uncertainty principle,” which comes in handy in quantum cryptography because the principle forbids simultaneously accurate measurements of certain kinds of paired variables, such as position and momentum. "But if you clone that system, you can measure one quantity in the first and the other quantity in the second, allowing you to decrypt an encoded message."

"In the presence of CTCs, quantum mechanics allows one to perform very powerful information-processing tasks, much more than we believe classical or even normal quantum computers could do," says Todd Brun, a physicist at the University of Southern California who was not involved with the team's experiment. "If the Deutsch model is correct, then this experiment faithfully simulates what could be done with an actual CTC. But this experiment cannot test the Deutsch model itself; that could only be done with access to an actual CTC."

Alternative reasoning Deutsch's model isn’t the only one around, however. In 2009 Seth Lloyd, a theorist at Massachusetts Institute of Technology, proposed an alternative , less radical model of CTCs that resolves the grandfather paradox using quantum teleportation and a technique called post-selection, rather than Deutsch's quantum self-consistency. With Canadian collaborators, Lloyd went on to perform successful laboratory simulations of his model in 2011. "Deutsch's theory has a weird effect of destroying correlations," Lloyd says. "That is, a time traveler who emerges from a Deutschian CTC enters a universe that has nothing to do with the one she exited in the future. By contrast, post-selected CTCs preserve correlations, so that the time traveler returns to the same universe that she remembers in the past."

This property of Lloyd's model would make CTCs much less powerful for information processing, although still far superior to what computers could achieve in typical regions of spacetime. "The classes of problems our CTCs could help solve are roughly equivalent to finding needles in haystacks," Lloyd says. "But a computer in a Deutschian CTC could solve why haystacks exist in the first place.”

Lloyd, though, readily admits the speculative nature of CTCs. “I have no idea which model is really right. Probably both of them are wrong,” he says. Of course, he adds, the other possibility is that Hawking is correct, “that CTCs simply don't and cannot exist." Time-travel party planners should save the champagne for themselves—their hoped-for future guests seem unlikely to arrive.

The Quantum Physics of Time Travel   (All-Access Subscribers Only) By David Deutsch and Michael Lockwood

Can Quantum Bayesianism Fix the Paradoxes of Quantum Mechanics?

Astrophysicist J. Richard Gott on Time Travel

• Search Menu
• Browse content in Arts and Humanities
• Browse content in Archaeology
• Anglo-Saxon and Medieval Archaeology
• Archaeological Methodology and Techniques
• Archaeology by Region
• Archaeology of Religion
• Archaeology of Trade and Exchange
• Biblical Archaeology
• Contemporary and Public Archaeology
• Environmental Archaeology
• Historical Archaeology
• History and Theory of Archaeology
• Industrial Archaeology
• Landscape Archaeology
• Mortuary Archaeology
• Prehistoric Archaeology
• Underwater Archaeology
• Urban Archaeology
• Zooarchaeology
• Browse content in Architecture
• Architectural Structure and Design
• History of Architecture
• Residential and Domestic Buildings
• Theory of Architecture
• Browse content in Art
• Art Subjects and Themes
• History of Art
• Industrial and Commercial Art
• Theory of Art
• Biographical Studies
• Byzantine Studies
• Browse content in Classical Studies
• Classical History
• Classical Philosophy
• Classical Mythology
• Classical Literature
• Classical Reception
• Classical Art and Architecture
• Classical Oratory and Rhetoric
• Greek and Roman Epigraphy
• Greek and Roman Law
• Greek and Roman Archaeology
• Greek and Roman Papyrology
• Late Antiquity
• Religion in the Ancient World
• Digital Humanities
• Browse content in History
• Colonialism and Imperialism
• Diplomatic History
• Environmental History
• Genealogy, Heraldry, Names, and Honours
• Genocide and Ethnic Cleansing
• Historical Geography
• History by Period
• History of Agriculture
• History of Education
• History of Emotions
• History of Gender and Sexuality
• Industrial History
• Intellectual History
• International History
• Labour History
• Legal and Constitutional History
• Local and Family History
• Maritime History
• Military History
• National Liberation and Post-Colonialism
• Oral History
• Political History
• Public History
• Regional and National History
• Revolutions and Rebellions
• Slavery and Abolition of Slavery
• Social and Cultural History
• Theory, Methods, and Historiography
• Urban History
• World History
• Browse content in Language Teaching and Learning
• Language Learning (Specific Skills)
• Language Teaching Theory and Methods
• Browse content in Linguistics
• Applied Linguistics
• Cognitive Linguistics
• Computational Linguistics
• Forensic Linguistics
• Grammar, Syntax and Morphology
• Historical and Diachronic Linguistics
• History of English
• Language Acquisition
• Language Variation
• Language Families
• Language Evolution
• Language Reference
• Lexicography
• Linguistic Theories
• Linguistic Typology
• Linguistic Anthropology
• Phonetics and Phonology
• Psycholinguistics
• Sociolinguistics
• Translation and Interpretation
• Writing Systems
• Browse content in Literature
• Bibliography
• Children's Literature Studies
• Literary Studies (Asian)
• Literary Studies (European)
• Literary Studies (Eco-criticism)
• Literary Studies (Modernism)
• Literary Studies (Romanticism)
• Literary Studies (American)
• Literary Studies - World
• Literary Studies (1500 to 1800)
• Literary Studies (19th Century)
• Literary Studies (20th Century onwards)
• Literary Studies (African American Literature)
• Literary Studies (British and Irish)
• Literary Studies (Early and Medieval)
• Literary Studies (Fiction, Novelists, and Prose Writers)
• Literary Studies (Gender Studies)
• Literary Studies (Graphic Novels)
• Literary Studies (History of the Book)
• Literary Studies (Plays and Playwrights)
• Literary Studies (Poetry and Poets)
• Literary Studies (Postcolonial Literature)
• Literary Studies (Queer Studies)
• Literary Studies (Science Fiction)
• Literary Studies (Travel Literature)
• Literary Studies (War Literature)
• Literary Studies (Women's Writing)
• Literary Theory and Cultural Studies
• Mythology and Folklore
• Shakespeare Studies and Criticism
• Browse content in Media Studies
• Browse content in Music
• Applied Music
• Dance and Music
• Ethics in Music
• Ethnomusicology
• Gender and Sexuality in Music
• Medicine and Music
• Music Cultures
• Music and Religion
• Music and Culture
• Music and Media
• Music Education and Pedagogy
• Music Theory and Analysis
• Musical Scores, Lyrics, and Libretti
• Musical Structures, Styles, and Techniques
• Musicology and Music History
• Performance Practice and Studies
• Race and Ethnicity in Music
• Sound Studies
• Browse content in Performing Arts
• Browse content in Philosophy
• Aesthetics and Philosophy of Art
• Epistemology
• Feminist Philosophy
• History of Western Philosophy
• Metaphysics
• Moral Philosophy
• Non-Western Philosophy
• Philosophy of Science
• Philosophy of Action
• Philosophy of Law
• Philosophy of Religion
• Philosophy of Language
• Philosophy of Mind
• Philosophy of Perception
• Philosophy of Mathematics and Logic
• Practical Ethics
• Social and Political Philosophy
• Browse content in Religion
• Biblical Studies
• Christianity
• East Asian Religions
• History of Religion
• Judaism and Jewish Studies
• Qumran Studies
• Religion and Education
• Religion and Health
• Religion and Politics
• Religion and Science
• Religion and Law
• Religion and Art, Literature, and Music
• Religious Studies
• Browse content in Society and Culture
• Cookery, Food, and Drink
• Cultural Studies
• Customs and Traditions
• Ethical Issues and Debates
• Hobbies, Games, Arts and Crafts
• Lifestyle, Home, and Garden
• Natural world, Country Life, and Pets
• Popular Beliefs and Controversial Knowledge
• Sports and Outdoor Recreation
• Technology and Society
• Travel and Holiday
• Visual Culture
• Browse content in Law
• Arbitration
• Browse content in Company and Commercial Law
• Commercial Law
• Company Law
• Browse content in Comparative Law
• Systems of Law
• Competition Law
• Browse content in Constitutional and Administrative Law
• Government Powers
• Judicial Review
• Local Government Law
• Military and Defence Law
• Parliamentary and Legislative Practice
• Construction Law
• Contract Law
• Browse content in Criminal Law
• Criminal Procedure
• Criminal Evidence Law
• Sentencing and Punishment
• Employment and Labour Law
• Environment and Energy Law
• Browse content in Financial Law
• Banking Law
• Insolvency Law
• History of Law
• Human Rights and Immigration
• Intellectual Property Law
• Browse content in International Law
• Private International Law and Conflict of Laws
• Public International Law
• IT and Communications Law
• Jurisprudence and Philosophy of Law
• Law and Politics
• Law and Society
• Browse content in Legal System and Practice
• Courts and Procedure
• Legal Skills and Practice
• Primary Sources of Law
• Regulation of Legal Profession
• Medical and Healthcare Law
• Browse content in Policing
• Criminal Investigation and Detection
• Police and Security Services
• Police Procedure and Law
• Police Regional Planning
• Browse content in Property Law
• Personal Property Law
• Study and Revision
• Terrorism and National Security Law
• Browse content in Trusts Law
• Wills and Probate or Succession
• Browse content in Medicine and Health
• Browse content in Allied Health Professions
• Arts Therapies
• Clinical Science
• Dietetics and Nutrition
• Occupational Therapy
• Operating Department Practice
• Physiotherapy
• Radiography
• Speech and Language Therapy
• Browse content in Anaesthetics
• General Anaesthesia
• Neuroanaesthesia
• Browse content in Clinical Medicine
• Acute Medicine
• Cardiovascular Medicine
• Clinical Genetics
• Clinical Pharmacology and Therapeutics
• Dermatology
• Endocrinology and Diabetes
• Gastroenterology
• Genito-urinary Medicine
• Geriatric Medicine
• Infectious Diseases
• Medical Oncology
• Medical Toxicology
• Pain Medicine
• Palliative Medicine
• Rehabilitation Medicine
• Respiratory Medicine and Pulmonology
• Rheumatology
• Sleep Medicine
• Sports and Exercise Medicine
• Clinical Neuroscience
• Community Medical Services
• Critical Care
• Emergency Medicine
• Forensic Medicine
• Haematology
• History of Medicine
• Browse content in Medical Dentistry
• Oral and Maxillofacial Surgery
• Paediatric Dentistry
• Restorative Dentistry and Orthodontics
• Surgical Dentistry
• Medical Ethics
• Browse content in Medical Skills
• Clinical Skills
• Communication Skills
• Nursing Skills
• Surgical Skills
• Medical Statistics and Methodology
• Browse content in Neurology
• Clinical Neurophysiology
• Neuropathology
• Nursing Studies
• Browse content in Obstetrics and Gynaecology
• Gynaecology
• Occupational Medicine
• Ophthalmology
• Otolaryngology (ENT)
• Browse content in Paediatrics
• Neonatology
• Browse content in Pathology
• Chemical Pathology
• Clinical Cytogenetics and Molecular Genetics
• Histopathology
• Medical Microbiology and Virology
• Patient Education and Information
• Browse content in Pharmacology
• Psychopharmacology
• Browse content in Popular Health
• Caring for Others
• Complementary and Alternative Medicine
• Self-help and Personal Development
• Browse content in Preclinical Medicine
• Cell Biology
• Molecular Biology and Genetics
• Reproduction, Growth and Development
• Primary Care
• Professional Development in Medicine
• Browse content in Psychiatry
• Addiction Medicine
• Child and Adolescent Psychiatry
• Forensic Psychiatry
• Learning Disabilities
• Old Age Psychiatry
• Psychotherapy
• Browse content in Public Health and Epidemiology
• Epidemiology
• Public Health
• Browse content in Radiology
• Clinical Radiology
• Interventional Radiology
• Nuclear Medicine
• Radiation Oncology
• Reproductive Medicine
• Browse content in Surgery
• Cardiothoracic Surgery
• Gastro-intestinal and Colorectal Surgery
• General Surgery
• Neurosurgery
• Paediatric Surgery
• Peri-operative Care
• Plastic and Reconstructive Surgery
• Surgical Oncology
• Transplant Surgery
• Trauma and Orthopaedic Surgery
• Vascular Surgery
• Browse content in Science and Mathematics
• Browse content in Biological Sciences
• Aquatic Biology
• Biochemistry
• Bioinformatics and Computational Biology
• Developmental Biology
• Ecology and Conservation
• Evolutionary Biology
• Genetics and Genomics
• Microbiology
• Molecular and Cell Biology
• Natural History
• Plant Sciences and Forestry
• Research Methods in Life Sciences
• Structural Biology
• Systems Biology
• Zoology and Animal Sciences
• Browse content in Chemistry
• Analytical Chemistry
• Computational Chemistry
• Crystallography
• Environmental Chemistry
• Industrial Chemistry
• Inorganic Chemistry
• Materials Chemistry
• Medicinal Chemistry
• Mineralogy and Gems
• Organic Chemistry
• Physical Chemistry
• Polymer Chemistry
• Study and Communication Skills in Chemistry
• Theoretical Chemistry
• Browse content in Computer Science
• Artificial Intelligence
• Computer Architecture and Logic Design
• Game Studies
• Human-Computer Interaction
• Mathematical Theory of Computation
• Programming Languages
• Software Engineering
• Systems Analysis and Design
• Virtual Reality
• Browse content in Computing
• Business Applications
• Computer Security
• Computer Games
• Computer Networking and Communications
• Digital Lifestyle
• Graphical and Digital Media Applications
• Operating Systems
• Browse content in Earth Sciences and Geography
• Atmospheric Sciences
• Environmental Geography
• Geology and the Lithosphere
• Maps and Map-making
• Meteorology and Climatology
• Oceanography and Hydrology
• Palaeontology
• Physical Geography and Topography
• Regional Geography
• Soil Science
• Urban Geography
• Browse content in Engineering and Technology
• Agriculture and Farming
• Biological Engineering
• Civil Engineering, Surveying, and Building
• Electronics and Communications Engineering
• Energy Technology
• Engineering (General)
• Environmental Science, Engineering, and Technology
• History of Engineering and Technology
• Mechanical Engineering and Materials
• Technology of Industrial Chemistry
• Transport Technology and Trades
• Browse content in Environmental Science
• Applied Ecology (Environmental Science)
• Conservation of the Environment (Environmental Science)
• Environmental Sustainability
• Environmentalist Thought and Ideology (Environmental Science)
• Management of Land and Natural Resources (Environmental Science)
• Natural Disasters (Environmental Science)
• Nuclear Issues (Environmental Science)
• Pollution and Threats to the Environment (Environmental Science)
• Social Impact of Environmental Issues (Environmental Science)
• History of Science and Technology
• Browse content in Materials Science
• Ceramics and Glasses
• Composite Materials
• Metals, Alloying, and Corrosion
• Nanotechnology
• Browse content in Mathematics
• Applied Mathematics
• Biomathematics and Statistics
• History of Mathematics
• Mathematical Education
• Mathematical Finance
• Mathematical Analysis
• Numerical and Computational Mathematics
• Probability and Statistics
• Pure Mathematics
• Browse content in Neuroscience
• Cognition and Behavioural Neuroscience
• Development of the Nervous System
• Disorders of the Nervous System
• History of Neuroscience
• Invertebrate Neurobiology
• Molecular and Cellular Systems
• Neuroendocrinology and Autonomic Nervous System
• Neuroscientific Techniques
• Sensory and Motor Systems
• Browse content in Physics
• Astronomy and Astrophysics
• Atomic, Molecular, and Optical Physics
• Biological and Medical Physics
• Classical Mechanics
• Computational Physics
• Condensed Matter Physics
• Electromagnetism, Optics, and Acoustics
• History of Physics
• Mathematical and Statistical Physics
• Measurement Science
• Nuclear Physics
• Particles and Fields
• Plasma Physics
• Quantum Physics
• Relativity and Gravitation
• Semiconductor and Mesoscopic Physics
• Browse content in Psychology
• Affective Sciences
• Clinical Psychology
• Cognitive Neuroscience
• Cognitive Psychology
• Criminal and Forensic Psychology
• Developmental Psychology
• Educational Psychology
• Evolutionary Psychology
• Health Psychology
• History and Systems in Psychology
• Music Psychology
• Neuropsychology
• Organizational Psychology
• Psychological Assessment and Testing
• Psychology of Human-Technology Interaction
• Psychology Professional Development and Training
• Research Methods in Psychology
• Social Psychology
• Browse content in Social Sciences
• Browse content in Anthropology
• Anthropology of Religion
• Human Evolution
• Medical Anthropology
• Physical Anthropology
• Regional Anthropology
• Social and Cultural Anthropology
• Theory and Practice of Anthropology
• Browse content in Business and Management
• Business Strategy
• Business History
• Business Ethics
• Business and Government
• Business and Technology
• Business and the Environment
• Comparative Management
• Corporate Governance
• Corporate Social Responsibility
• Entrepreneurship
• Health Management
• Human Resource Management
• Industrial and Employment Relations
• Industry Studies
• Information and Communication Technologies
• International Business
• Knowledge Management
• Management and Management Techniques
• Operations Management
• Organizational Theory and Behaviour
• Pensions and Pension Management
• Public and Nonprofit Management
• Strategic Management
• Supply Chain Management
• Browse content in Criminology and Criminal Justice
• Criminal Justice
• Criminology
• Forms of Crime
• International and Comparative Criminology
• Youth Violence and Juvenile Justice
• Development Studies
• Browse content in Economics
• Agricultural, Environmental, and Natural Resource Economics
• Asian Economics
• Behavioural Finance
• Behavioural Economics and Neuroeconomics
• Econometrics and Mathematical Economics
• Economic Systems
• Economic Methodology
• Economic History
• Economic Development and Growth
• Financial Markets
• Financial Institutions and Services
• General Economics and Teaching
• Health, Education, and Welfare
• History of Economic Thought
• International Economics
• Labour and Demographic Economics
• Law and Economics
• Macroeconomics and Monetary Economics
• Microeconomics
• Public Economics
• Urban, Rural, and Regional Economics
• Welfare Economics
• Browse content in Education
• Adult Education and Continuous Learning
• Care and Counselling of Students
• Early Childhood and Elementary Education
• Educational Equipment and Technology
• Educational Strategies and Policy
• Higher and Further Education
• Organization and Management of Education
• Philosophy and Theory of Education
• Schools Studies
• Secondary Education
• Teaching of a Specific Subject
• Teaching of Specific Groups and Special Educational Needs
• Teaching Skills and Techniques
• Browse content in Environment
• Applied Ecology (Social Science)
• Climate Change
• Conservation of the Environment (Social Science)
• Environmentalist Thought and Ideology (Social Science)
• Natural Disasters (Environment)
• Social Impact of Environmental Issues (Social Science)
• Browse content in Human Geography
• Cultural Geography
• Economic Geography
• Political Geography
• Browse content in Interdisciplinary Studies
• Communication Studies
• Museums, Libraries, and Information Sciences
• Browse content in Politics
• African Politics
• Asian Politics
• Chinese Politics
• Comparative Politics
• Conflict Politics
• Elections and Electoral Studies
• Environmental Politics
• European Union
• Foreign Policy
• Gender and Politics
• Human Rights and Politics
• Indian Politics
• International Relations
• International Organization (Politics)
• International Political Economy
• Irish Politics
• Latin American Politics
• Middle Eastern Politics
• Political Methodology
• Political Communication
• Political Philosophy
• Political Sociology
• Political Theory
• Political Behaviour
• Political Economy
• Political Institutions
• Politics and Law
• Public Administration
• Public Policy
• Quantitative Political Methodology
• Regional Political Studies
• Russian Politics
• Security Studies
• State and Local Government
• UK Politics
• US Politics
• Browse content in Regional and Area Studies
• African Studies
• Asian Studies
• East Asian Studies
• Japanese Studies
• Latin American Studies
• Middle Eastern Studies
• Native American Studies
• Scottish Studies
• Browse content in Research and Information
• Research Methods
• Browse content in Social Work
• Addictions and Substance Misuse
• Adoption and Fostering
• Care of the Elderly
• Child and Adolescent Social Work
• Couple and Family Social Work
• Developmental and Physical Disabilities Social Work
• Direct Practice and Clinical Social Work
• Emergency Services
• Human Behaviour and the Social Environment
• International and Global Issues in Social Work
• Mental and Behavioural Health
• Social Justice and Human Rights
• Social Policy and Advocacy
• Social Work and Crime and Justice
• Social Work Macro Practice
• Social Work Practice Settings
• Social Work Research and Evidence-based Practice
• Welfare and Benefit Systems
• Browse content in Sociology
• Childhood Studies
• Community Development
• Comparative and Historical Sociology
• Economic Sociology
• Gender and Sexuality
• Gerontology and Ageing
• Health, Illness, and Medicine
• Marriage and the Family
• Migration Studies
• Occupations, Professions, and Work
• Organizations
• Population and Demography
• Race and Ethnicity
• Social Theory
• Social Movements and Social Change
• Social Research and Statistics
• Social Stratification, Inequality, and Mobility
• Sociology of Religion
• Sociology of Education
• Sport and Leisure
• Urban and Rural Studies
• Browse content in Warfare and Defence
• Defence Strategy, Planning, and Research
• Land Forces and Warfare
• Military Administration
• Military Life and Institutions
• Naval Forces and Warfare
• Other Warfare and Defence Issues
• Peace Studies and Conflict Resolution
• Weapons and Equipment

• < Previous chapter
• Next chapter >

13 Paradoxes of Time Travel to the Future

• Published: July 2022
• Cite Icon Cite
• Permissions Icon Permissions

This chapter brings two fresh perspectives on Lewis’s theory of time travel. First: many key aspects and theoretical desiderata of Lewis’s theory can be captured in a framework that does not commit to eternalism about time. Second: implementing aspects of Lewisian time travel in a non-eternalist framework provides theoretical resources for a better treatment of time travel to the future. While time travel to the past has been extensively analyzed, time travel to the future has been comparatively underexplored. The chapter makes progress on this topic. Along the way, it discusses Lewis’s lesser-known time travel oeuvre, especially his recently published correspondence and lectures on the topic. Lewis’s body of unpublished work on time travel yields fruitful insights into his broader thinking on the subject.

Signed in as

Institutional accounts.

• Google Scholar Indexing
• GoogleCrawler [DO NOT DELETE]

Personal account

• Sign in with email/username & password
• Get email alerts
• Save searches
• Purchase content
• Activate your purchase/trial code

Institutional access

• Sign in with a library card Sign in with username/password Recommend to your librarian
• Institutional account management
• Get help with access

Access to content on Oxford Academic is often provided through institutional subscriptions and purchases. If you are a member of an institution with an active account, you may be able to access content in one of the following ways:

IP based access

Typically, access is provided across an institutional network to a range of IP addresses. This authentication occurs automatically, and it is not possible to sign out of an IP authenticated account.

Sign in through your institution

Choose this option to get remote access when outside your institution. Shibboleth/Open Athens technology is used to provide single sign-on between your institution’s website and Oxford Academic.

• Click Sign in through your institution.
• Select your institution from the list provided, which will take you to your institution's website to sign in.
• When on the institution site, please use the credentials provided by your institution. Do not use an Oxford Academic personal account.
• Following successful sign in, you will be returned to Oxford Academic.

If your institution is not listed or you cannot sign in to your institution’s website, please contact your librarian or administrator.

Sign in with a library card

Enter your library card number to sign in. If you cannot sign in, please contact your librarian.

Society Members

Society member access to a journal is achieved in one of the following ways:

Sign in through society site

Many societies offer single sign-on between the society website and Oxford Academic. If you see ‘Sign in through society site’ in the sign in pane within a journal:

• Click Sign in through society site.
• When on the society site, please use the credentials provided by that society. Do not use an Oxford Academic personal account.

If you do not have a society account or have forgotten your username or password, please contact your society.

Sign in using a personal account

Some societies use Oxford Academic personal accounts to provide access to their members. See below.

A personal account can be used to get email alerts, save searches, purchase content, and activate subscriptions.

Some societies use Oxford Academic personal accounts to provide access to their members.

Viewing your signed in accounts

Click the account icon in the top right to:

• View your signed in personal account and access account management features.
• View the institutional accounts that are providing access.

Signed in but can't access content

Oxford Academic is home to a wide variety of products. The institutional subscription may not cover the content that you are trying to access. If you believe you should have access to that content, please contact your librarian.

For librarians and administrators, your personal account also provides access to institutional account management. Here you will find options to view and activate subscriptions, manage institutional settings and access options, access usage statistics, and more.

Our books are available by subscription or purchase to libraries and institutions.

• About Oxford Academic
• Publish journals with us
• University press partners
• What we publish
• New features  
• Open access
• Rights and permissions
• Accessibility
• Advertising
• Media enquiries
• Oxford University Press
• Oxford Languages
• University of Oxford

Oxford University Press is a department of the University of Oxford. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide

• Copyright © 2024 Oxford University Press
• Cookie settings
• Cookie policy
• Privacy policy
• Legal notice

This Feature Is Available To Subscribers Only

Sign In or Create an Account

This PDF is available to Subscribers Only

For full access to this pdf, sign in to an existing account, or purchase an annual subscription.

Time Travel Movies Rely on the ‘Bootstrap Paradox.’ It Could Explain Real-Life Destiny

It may not be as famous as the so-called “grandfather paradox,” but that doesn’t make the idea of an infinite causal loop any less troubling ... or fascinating .

Arriving back slightly before your time, you watch your friend as she teaches her younger self how to build the time machine in the first place. “But … ” you stammer, cautious in case your friend—or her younger counterpart—takes offense and travels back in time to wipe you from existence, “if you used the time machine to travel back in time to inform yourself how to build the time machine, isn’t that a paradox?” It is , because no one in this story ever actually sat down to figure out how to create a time machine. This is the bootstrap paradox, which details an infinite cause-and-effect loop that develops around information, an object, or even a person that can no longer be given a discernible point of origin in a time travel scenario.

This results in a “closed causal loop,” in which some event or object is a key player in its own origin and couldn’t exist without its later self. The idea is woven through the imaginations of science fiction writers, theoretical physicists, and philosophers, meaning its roots in science and pop culture may be irrevocably interwoven.

Though less famous than the grandfather paradox —in which a time traveler kills their grandfather before they have children, and can no longer exist to go back in time and perform grand patricide—the bootstrap paradox is still an idea that philosophers and physicists have been grappling with for almost 100 years.

“The bootstrap paradox in time travel occurs when a piece of information that has no right to exist, nonetheless exists,” Seth Lloyd, a professor of mechanical engineering at the Massachusetts Institute of Technology and a self-described “quantum mechanic,” tells Popular Mechanics . “To illustrate the bootstrap paradox, consider the version known as the ‘unproved theorem paradox.’”

Lloyd explains that in the unproved theorem paradox, a time traveler finds a beautiful and elegant proof of a theorem in a book, then goes back in time and shows the proof to a mathematician, who includes the proof in his book. “The book, of course, is the same book in which the time traveler found the proof in the future,” he adds. “So who proved the theorem? No one proved the theorem! Where did the proof come from? Nowhere!”

Tim Maudlin, a philosopher of science who investigates the metaphysical foundations of physics and logic, says there is a key difference between the bootstrap paradox and the grandfather paradox.

“The bootstrap paradox occurs in certain logically consistent time travel scenarios. Unlike the grandfather paradox, which is a self-contradictory and hence an impossible scenario, in a bootstrap story, everything fits together in a logically coherent way,” Maudlin tells Popular Mechanics. “Still, the paradox arises because certain things or information [are] ‘self-caused’ or ‘come from itself.’” That means, in a sense, that these bits of information appear in the story without ever having been created, just like the instructions for building your friend’s time machine.

Maudlin — who alongside Oxford University philosopher Frank Arntzenius wrote the Stanford Encyclopedia of Philosophy article “Time Travel and Modern Physics”— adds that since it is not a matter of self-contradiction or logical incoherence, logic alone cannot rule out these bootstrap paradox scenarios. This has helped make the concept a staple of time travel stories in popular culture.

The Origins of the Bootstrap Paradox

Physicists first began to pay serious attention to the concept of time travel in the early years of the 20th century, following the work of Albert Einstein .

As he formulated the theory of special relativity, published in 1905, Einstein united the concepts of space and time into a single four-dimensional entity — spacetime — and also imposed the rule that particles with mass would need infinite energy to accelerate to the speed of light . This led to speculation about hypothetical massless particles called “tachyons” that could exceed the speed of light, but as a consequence, would travel back through time.

.css-2l0eat{font-family:UnitedSans,UnitedSans-roboto,UnitedSans-local,Helvetica,Arial,Sans-serif;font-size:1.625rem;line-height:1.2;margin:0rem;padding:0.9rem 1rem 1rem;}@media(max-width: 48rem){.css-2l0eat{font-size:1.75rem;line-height:1;}}@media(min-width: 48rem){.css-2l0eat{font-size:1.875rem;line-height:1;}}@media(min-width: 64rem){.css-2l0eat{font-size:2.25rem;line-height:1;}}.css-2l0eat b,.css-2l0eat strong{font-family:inherit;font-weight:bold;}.css-2l0eat em,.css-2l0eat i{font-style:italic;font-family:inherit;} “The paradox suggests the potential for ‘predestination’ to causality, or the idea that the past is dependent on the future.”

While this opened the door to physicists thinking about time travel, Einstein would violently tear that hypothetical door down ten years later when he formulated the theory of general relativity . Primarily a theory that describes the effect of mass on spacetime, general relativity implies that four-dimensional spacetime could be twisted into any shape. This means it could be possible to create a path that forms a loop passing back through time, returning to its original starting point — which would become known as a “closed time-like curve” or CTC.

The idea of time travel caused a serious threat to causality, the relationship between cause and effect, in that it could make an effect precede its own cause—a clear contradiction. The bootstrap paradox is more subtle than this (cause still precedes effect), but there is no starting point in the process. Instead, the paradox suggests the potential for “predestination” to causality, or the idea that the past is dependent on the future.

As the bootstrap paradox worked its way into physicists’ notebooks and blackboards, it was also making its mark on pop culture, with the two occasionally overlapping.

The Bootstrap Paradox in Pop Culture

The bootstrap paradox takes its name from an expression that dates back to 1834: to “pull oneself over a fence by one’s bootstraps,” which came to represent the idea of performing a ludicrous or impossible task. (In the startup world, it’s come to mark the idea of launching a company with your own money or whatever you can scrounge from friends and family—certainly a Herculean task.)

In popular culture, the paradox lent its name to the 1941 Robert A. Heinlein story, By His Bootstraps , likely one of the paradox’s earliest presentations to the public. In the story, a similar scenario plays out to the one we described about your time-traveling friend:

“A time travel machine is built by following the instructions in a book, which itself is sent back from the future. But how did the information get into the book? By being copied into a newly made notebook from the older version of itself,” Maudlin explains. “That is, no one ever sat down and figured out how to build a time machine, the information about how to do it is just there in a closed causal loop.”

Since the publication of Heinlein’s tale, the bootstrap paradox has made its way into a wealth of science fiction tales, though it often goes unnamed. For instance, in the popular Doctor Who episode “Blink,” the time-traveler that David Tennett plays must combat a menace called the Weeping Angels by passing messages from the past—when he is trapped without his own method of time travel, the TARDIS—to the present.

At the end of the story, with evil defeated, a log of the messages is passed back to the timelord before he is trapped in the past. Therefore, the log of messages has no set origin; It cycles between the past and the future in a closed causal loop.

Meanwhile, the Terminator franchise features a wealth of potential time travel paradoxes across the series. For example, the main mission of the earliest film is for Arnold Schwarzenegger’s T-800 to prevent the birth of John Connor. Yet, if the Terminator kills his mother Sarah, and prevents the birth of John Conner before he can lead the rebellion, then he never existed as a threat to the Terminator controlling Skynet in the first place; That means they’d never need to send a Terminator back to kill him at all,eEssentially creating a version of the Grandfather paradox with more robots.

Artificial intelligence-based boogeyman Skynet is itself, however, an example of the bootstrap paradox. The AI system that would go on to rule Earth and subjugate mankind was created from the leftover parts of the T-800 Terminator that was sent back in time to kill Sarah Connor and prevent the birth of John Connor in the first place. Eventually, Skynet’s own killing machines, including the T-800, were retro-engineered from these parts, meaning the T-800 is the source of its own origin. Classic bootstrap!

These are just a few of the more infamous examples of the bootstrap paradox in fiction, and some of these tales have even helped inform scientific theory on the subject.

“As part of our efforts to construct a self-consistent quantum theory of time travel via closed timelike curves, I did extensive reading of time travel narratives and viewing of time travel films,” Lloyd explains. “All time travel paradoxes seem to be versions of either the grandfather paradox or of the bootstrap paradox.”

Escaping the Bootstrap Paradox

Hugh Everett’s Many-Worlds Interpretation of quantum mechanics suggests that for every quantum possibility, a distinct universe is created. This can provide a solution to apparent contradictions in time travel.

“It has been suggested that paradoxes such as the grandfather paradox could be avoided by Everett’s Many Worlds—or more accurately, ‘relative state’—interpretation of quantum mechanics ,” Lloyd says. “When the time traveler goes back and kills her grandfather, she is simply forcing a transition into a separate ‘world,’ or branch of the wave function, from the world in which she came.”

That means a time traveler hitching a ride on a closed time-like curve into the past actually causes the universe to diverge, and the world she arrives in is distinct from the one she left. Therefore, it is not her grandfather she kills in the past, but a version of him that will never go on to have a child or grandchild. Our time traveler’s birth, therefore, is not prevented.

So in the case of the bootstrap paradox, returning to our earlier scenario, our friend doesn’t pass the information for building a time machine to her earlier self, but a version of herself in the past in a separate world.

That means each backward leap passes the information to a new universe.

Even so, this doesn’t fully solve the Bootstrap paradox, as it doesn’t explain where this information came from in the first place; The instructions for the time travel machine still don’t have an origin point.

“Since no contradiction is involved, one can’t raise the same logical objection as in the grandfather paradox case,” Maudlin says. “But still, many people think that such ‘self-causation’ or lack of an originator of the information, makes the scenario impossible. And of course, if reverse causation is not possible, then this closed causal loop is not either.”

Maudlin does point out that the bootstrap paradox isn’t as intrinsically intractable as the grandfather paradox, however. In fact, it could be replete with a wealth of hitherto undiscovered exit routes.

“If one allows for closed causal loops, then in a way, the opposite thing happens as in the grandfather paradox case. With the grandfather paradox, there seems to be no logically self-consistent solution,” Maudlin concludes. “But in the bootstrapping cases, there are often several different solutions, with the closed loops taking different forms.

“In fact, there would not seem to be any explanation of why one, rather than another, of these solutions would be realized.”

Testing the Bootstrap Paradox

While Maudlin doesn’t believe that the bootstrap paradox could ever be experimentally tested, Lloyd disagrees; He has suggested an experiment to investigate the unproved theorem paradox version of the bootstrap paradox.

“We devised experiments to test the pre/retrodictions of our theory for both of the central paradoxes of time travel—the grandfather paradox and the bootstrap paradox—a simple experiment to test the pre/retrodictions of our theory for the unproved theorem paradox,” Lloyd says. “The test can be performed on a simple quantum computer . In the experiment, a bit of information—call it Bit A—is copied to a second bit (Bit B).

“Bit B is sent back into the past, where it turns out that B in the past is in fact the same bit that becomes A in the future. Rather than engendering a self-inconsistency, however, the bootstrap/unproved theorem experiment is entirely self-consistent.”

This means for Lloyd, perhaps the greatest paradox surrounding the bootstrap paradox is that it doesn’t have to be a paradox at all.

“The bootstrap paradox isn’t always paradoxical. In our proposed experiment, for example, we predict that the ‘unproved’ bit will turn out to be entirely random,” he says. “This makes sense, as at no point in the future or the past was there any bias introduced to make it anything other than random .”

For now, however, Lloyd doesn’t intend to attempt such an experiment. Instead, alongside his colleague Michele Reilly and artist Andrey Kezzyn, Lloyd expressed his interest in time travel in the form of the 2022 movie Steeplechase .

“The experimental demonstration of the grandfather paradox gave rise to so much publicity—not to say notoriety—however, that we decided to hold off on further time travel experiments for the foreseeable future,” the physicist explains.

Robert Lea is a freelance science journalist focusing on space, astronomy, and physics. Rob’s articles have been published in Newsweek , Space , Live Science , Astronomy magazine and New Scientist . He lives in the North West of England with too many cats and comic books.  

.css-cuqpxl:before{padding-right:0.3125rem;content:'//';display:inline;} Physics .css-xtujxj:before{padding-left:0.3125rem;content:'//';display:inline;}

Is Consciousness Everywhere All at Once?

You’ll Never Live Forever—Blame Physics

One Particle Could Shatter Our Concept of Reality

Do Black Holes Die?

Are Multiverse Films Like ‘The Flash’ Realistic?

Why Time Reflections Are a ‘Holy Grail’ in Physics

Why Our Existence Always Contains Some Uncertainty

Copies of You Could Live Inside Quantum Computers

There’s an ‘Anti-Universe’ Going Backward in Time

The Weird Double Slit Experiment Just Got Weirder

How Fast Is Warp Speed, Hypothetically Speaking?

• [ November 30, 2022 ] The Night Sky This Month: December 2022 Night Sky
• [ November 22, 2022 ] James Webb Telescope Turns Its Attention To The Kuiper Belt News & Events
• [ November 1, 2022 ] The Night Sky This Month: November 2022 Night Sky
• [ October 4, 2022 ] Are Wormholes Fact or Fiction? General Astronomy
• [ October 1, 2022 ] The Night Sky This Month: October 2022 Night Sky

Time Travel & the Bootstrap Paradox Explained

October 17, 2015 James Miller Time Travel 21

The Bootstrap Paradox is a theoretical paradox of time travel that occurs when an object or piece of information sent back in time becomes trapped within an infinite cause-effect loop in which the item no longer has a discernible point of origin, and is said to be “uncaused” or “self-created”. It is also known as an Ontological Paradox, in reference to ontology, a branch of metaphysics dealing with the study of being and existence.

Etymology of Bootstrap Paradox

The term Bootstrap Paradox is derived from the expression to “pull oneself over a fence by one’s bootstraps”, which indicates performing an impossible or ludicrous task. In this instance, by pulling yourself over a fence by holding onto your bootlaces and tugging upwards. The first reference to such an absurdly impossible action is widely believed to originate from an 18th-century literary classic, ‘ The Surprising Adventures of Baron Munchhausen’ , in which the eponymous hero is stuck in a swamp, and manages to escape by pulling upwards on his own hair.

The term “bootstrap paradox” was subsequently popularized by science fiction writer Robert A. Heinlein, whose book, ‘ By His Bootstraps’ (1941), tells the story of Bob Wilson, and the time travel paradoxes he encounters after using a time portal. One such example involves Wilson traveling to the future and being given a notebook by his future self, before then traveling to an earlier point in the future and using the book’s useful information to set himself up as a benevolent dictator. After the notebook becomes worn, Wilson copies the information into a new notebook and disposes of the original. He later muses that there never were two notebooks and that the newly created one is actually the one given to him in the far future. So who wrote the book, and where did its information actually originate?

Bootstrap Paradox Examples

– Information: An example of a bootstrap paradox involving information would be if a time traveler went back in time and taught Einstein the theory of relativity , before returning to his own time. Einstein claims it’s his own work, and over the following decades the theory is published countless times until a copy of it eventually ends up in the hands of the original time traveler who then takes it back to Einstein, begging the question “where did the theory originate”. We cannot say that it came from the time traveler as he learned it from Einstein, but we also cannot say that it is from Einstein, since he was taught it by the time traveler . Who, then, discovered the theory of relativity?

In fiction, the Doctor Who episode ‘ Blink ‘ contains an information paradox in which a video message forms an endless loop spanning thirty-eight years. Likewise, the two-part Doctor Who episodes ‘ Under the Lake ‘, and ‘ Before the Flood ‘ also features a nifty paradox anecdote involving Beethoven’s music. The 2014 film ‘ Time Lapse ‘ provides a further example of a story rich in bootstrap paradoxes, with the main characters responding daily to photos they receive from 24 hours into their future.

– Object: The 1980’s movie Somewhere in Time provides an example of a bootstrap paradox involving an object, in this case a pocket watch. In 1972, Christopher Reeve is given a watch by an old woman, which it turns out was given to her younger self by Reeve after traveling back to 1912. The young woman then completes the infinite loop by giving the watch to Reeve in 1972 when she’s older. An inconsistency that subsequently arises is how the pocket watch survives countless time cycles while remaining “unaged” and unaffected by time. The problem is no less true for information trapped inside a bootstrap paradox. Both seem to violate the second-law of Thermodynamics, which states that entropy (gradual decline into disorder) will always increase over time.

In the Terminator movies , Skynet is an example of a bootstrap paradox involving an object. Skynet, the conscious AI system and mankind’s nemesis, could not have been invented without the leftover components of the T-800 cybernetic organism it sent back in time to stop John Connor. The technology was analyzed and Skynet and cyborgs were subsequently created through reverse engineering.

– Person: The most extreme example of a bootstrap paradox involving a person can be found in the Robert A. Heinlein’s short story “ All You Zombies ” (1959), which inspired the 2014 movie “ Predestination “. Here the main character, an intersex male born a female, is tricked into going back in time and impregnating his pre-gender reassigned female self, who subsequently gives birth to himself/herself. As a result, he becomes a self-created entity who is both his own mother and father. This naturally presents a real mind-bending chicken-and-egg conundrum. Once again, however, the story appears to be self-consistent, with no changes taking place each time through the loop. Nevertheless, Heinlein doesn’t attempt to answer the role “free will” plays in this imaginative scenario.

The Futurama episode ‘ Roswell That Ends Well ‘ where Fry becomes his own grandfather provides another good example of a person-centric bootstrap paradox in fiction. As does the Terminator movies, once more, with a future John Connor sending Kyle Reese to the past to impregnate Sarah Connor, who then gives birth to John Connor.

Self-Consistent with Timeline

Consistency Paradoxes, such as the Grandfather Paradox , The Hitler paradox, and Polchinski’s Paradox, result in a ‘self-inconsistent’ solution with the timeline’s history. After all, if a time traveler killed his own grandfather then he would never have been born, and so would not have been able to travel back through time and murder his grandfather. This would be a paradox.

The Predestination Paradox and the Bootstrap Paradox, on the other hand, are examples of closed loops in time in which ’cause and effect’ repeat in a circular pattern, resulting in a self-created entity with no point of origin. Despite being an oddity and apparently conspiring against our understanding of causality, this ‘self-caused’ event, like the Big Bang, does not appear to be an impossibility. Nor does it imply any inconsistency with the timeline’s history. In fact, all the events in the time loop are “fixed” and take place on a single unchangeable timeline.

Einstein’s Theory of General Relativity tells us that we have got almost complete freedom of movement into the future. Time travel to the past, on the other hand, throws up a number of paradoxes. That’s despite his equations maintaining that four-dimensional space-time can be twisted into any shape, and that loops in space-time are possible. Any time travel paradoxes that do arise are therefore of particular concern to theoretical physicists. Their line of reasoning has subsequently led many of them to conclude that time travel to the past must be impossible. Some of those fundamental breaches in the laws of physics include the following examples:

– Law of Causality: While a bootstrap paradox may produce a consistent account of the timeline’s history, one problem associated with this ontological conundrum is an apparent violation of the Law of Causality. As a result, scientists are presented with an obvious problem in that they are no longer able to say that a past ’cause’ leads to a future ‘event’. After all, the event may equally have been created in the future before leading to its cause in the past. This suggests that instead of time moving from a dead past to an undetermined future, the past, present, and future are, in fact, all equally real at the same time. In the process, rendering the task of defining the “origin” of anything, a term usually associated with the past, now meaningless.

– Law of Entropy: Another problem associated with a bootstrap paradox is an apparent violation of the second law of thermodynamics , which states that systems always flow from a state of order to a state of disorder. This would suggest that an object or information trapped within a time loop would continue to age and eventually disintegrate. We touched upon this earlier with the pocket watch in Somewhere In Time, which one would have expected to get older as it progressed through the cycle. In which case, the item cannot be the same as the one sent back in time, which creates a contradiction and raises the prospect of Theseus’ paradox , and the question of identity. Furthermore, the watch ultimately wearing out would also indicate a discontinuity in the story, as Jane Seymour could then have never have received it as a young woman and the time loop could never have started.

Possible Solutions

Working on the assumption of an “immutable” timeline in which the circle of events are identical every time, the ‘Somewhere In Time’ example raises the problem of an increasingly aging pocket watch. One solution may be to assume that entropy is somehow reversed by time travel, although this may also suggest that the matter which comprised Reeve himself would also have subsequently been restored to its 1912 state when he returned to the past, which needless to say would not be in the form of Reeve.

Well, perhaps not, according to Russian professor Novikov, as the second law of thermodynamics is thought to be a statistical law, and not an absolute one, making spontaneous entropy reversals or failure to increase improbable, but not impossible. Furthermore, the second law of thermodynamics applies only to a system isolated from the external world, and as Novikov argues:

“.. in the case of macroscopic objects like the watch whose worldlines form closed loops, the outside world can expend energy to repair wear/entropy that the object acquires over the course of its history, so that it will be back in its original condition when it closes the loop. ( wiki )”

Otherwise, it would be intriguing to consider the possibility that the time-traveling watch might have to obey the ‘timeline protection hypothesis’ which states that any attempt to create a paradox would fail due to a probability distortion being created. Imagine a young Jane Seymour becoming angry, for instance, and throwing the watch at the wall. The wall may be damaged slightly but the watch must remain in the same state. Probability would bend to prevent any damage occurring to the watch, which could result in some pretty incredible outcomes. Nevertheless, the universe must favor an improbable event happening, in order to prevent an impossible one.

A final possibility involves a chrononaut finding himself in a parallel universe or multiverse each time he travels to the past, thereby changing nothing of his original timeline.

Still hungry to explore further the nature of time and all its mysteries ? Simply follow the link opposite.

Related Posts

© Copyright 2023 Astronomy Trek

The Time-Travel Paradoxes

What happens if a time traveler kills his or her grandfather? What is a time loop? How do you stop a time machine from just appearing somewhere in space, millions of kilometers from home? And is there such a thing as free will?

Congratulations! You have a time machine! You can pop over to see the dinosaurs, be in London for the Beatles’ rooftop concert, hear Jesus deliver his Sermon on the Mount, save the books of the Library of Alexandria, or kill Hitler. Past and future are in your hands. All you have to do is step inside and press the red button.

Wait! Don’t do it!

Seriously, if you value your lives, if you want to protect the fabric of reality – run for the hills! Physics and logical paradoxes will be your undoing. From the grandfather paradox to laws of classic mechanics, we have prepared a comprehensive guide to the hazards of time travel. Beware the dangers that lie ahead.

 The Grandfather Paradox

Want to change reality? First think carefully about your grandparents’ contribution to your lives.

The grandfather paradox basically describes the following situation: For some reason or another, you have decided to go back in time and kill your grandfather in his youth. Yeah, sure, of course you love him – but this is a scientific experiment; you don’t have a choice. So your grandmother will never give birth to your parent – and therefore you will never be born, which means that you cannot kill your grandfather. Oh boy! This is quite a contradiction!

The extended version of the paradox touches upon practically every single change that our hypothetical time traveler will make in the past. In a chaotic reality, there is no telling what the consequences of each step will be on the reality you came from. Just as a butterfly flapping its wings in the Amazon could cause a tornado in Texas, there is no way of predicting what one wrong move on your part might do to all of history, let alone a drastic move like killing someone.

There is a possible solution to this paradox – but it cancels out free will: Our time traveler can only do what has already been done. So don’t worry – everything you did in the past has already happened, so it’s impossible for you to kill grandpa, or create any sort of a contradiction in any other way. Another solution is that the time traveler's actions led to a splitting of the universe into two universes – one in which the time traveler was born, and the other in which he murdered his grandfather and was not born.

Information passage from the future to the past causes a similar paradox. Let’s say someone from the future who has my best interests in mind tries to warn me that a grand piano is about to fall on my head in the street, or that I have a type of cancer that is curable if it’s discovered early enough. Because of this warning, I could take steps to prevent the event – but then, there is no reason to send back the information from the future that saves my life. Another contradiction!

Marty finds himself in hot water with the grandfather paradox, from ‘Back to the Future’ 1985

Let’s now assume the information is different: A richer future me builds a time machine to let the late-90s me know that I should buy stock of a small company called “Google”, so that I can make a fortune. If I have free will, that means I can refuse. But future me knows I already did it. Do I have a choice but to do what I ask of myself?

 The Time Loop

In the book All You Zombies by science fiction writer Robert A. Heinlein the Hero is sent back in time in order to impregnate a young woman who is later revealed to be him, following a sex change operation. The offspring of this coupling is the young man himself, who will meet himself at a younger age and take him back to the past to impregnate you know whom.

Confused? This is just one extreme example of a time loop – a situation where a past event is the cause of an event at another time and also the result of it. A simpler example could be a time traveler giving the young William Shakespeare a copy of the complete works of Shakespeare so that he can copy them. If that happens, then who is the genius author of Macbeth?

This phenomenon is also known as the Bootstrap Paradox , based on another story by Heinlein, who likened it to a person trying to pull himself up by his bootstraps (a phrase which, in turn, comes from the classic book The Surprising Adventures of Baron Munchausen). The word ‘paradox’ here is a bit misleading, since there is no contradiction in the loop – it exists in a sequence of events and feeds itself. The only contradiction is in the order of things that we are acquainted with, where cause leads to effect and nothing further, and there is meaning to the question “how did it all begin?”

 Terminator 2 (1991). The shapeshifting android (Arnold Schwarzenegger) destroys himself in order to break the time loop in which his mere presence in the present enabled his production in the future

Time travelers – where have all they gone?

In 1950, over lunch physicist Enrico Fermi famously asked: “If there is intelligent extraterrestrial life in the Universe – then where are they?” indicating that we have never met aliens or came across evidence of their existence, such as radio signals which would be proof of a technological society.  We could pose that same question about time travelers: “If time travel is possible, where are all the time travelers?”

The question, known as the Fermi Paradox, is an important one. After all, if it were possible to travel through time, would we not have bumped into a bunch of observers from the future at critical junctures in history? It is unlikely to assume that they all managed to perfectly disguise themselves, without making any errors in the design of the clothes they wore, their accents, their vocabulary, etc. Another option is that time travel is possible, but it is used with the utmost care and tight control, due to all the dangers we discuss here.

 On June 28, 2009, physicist Stephen Hawking carried out a scientific experiment which was meant to answer this question once and for all. He brought snacks, balloons and champagne and hosted a secret party for time travelers only – but sent out the invitations only on the next day. If no one showed up, he argued, that would be proof that time travel to the past is not possible. The invitees failed to arrive. “I sat and waited for a while, but nobody came,” he reported at the Seattle Science Festival in 2012.

Multiple time travelers also undermine the possibility of a fixed and consistent timeline, assuming that the past can indeed be changed. Imagine, for example, a nail-biting derby between the top clubs, Hapoel Jericho and Maccabi Jericho. Originally Maccabi won, so a Hapoel fan traveled back in time and managed to lead to his team’s victory. Maccabi fans would not give up and did the same. Soon, the whole stadium is filled with time travelers and paradoxes.

 One way or round trip?

When considering travel, it is always continuous – from point A to point B, through all the points in between. Time travel should supposedly be the same: travelers get into their machine, push the button, and go from time A to time B, through all the times in between. But there’s a catch, if we are only travelling through time, then to the casual observer, the time machine continuously exists in the same space between the points in time. The result is that our journey is one-way and the time travelers will stay stuck in the future or the past because the machine itself will block the time-path back. And that is before we even start wondering how to even build this thing in the first place if it already exists in the place where we want to build it.

If that’s the case, then there’s no choice but to assume that there is some way to jump from time to time or place to place and materialize at the destination. How will our machine “know” to jump to an empty area, and to avoid materializing into a wall or a living creature unlucky enough to occupy that same spot? The passengers will undoubtedly need effective navigation and observation equipment to prevent unfortunate accidents at the point of entry.

 Advanced time travel

In addition to the problems that time travel poses for anyone trying to keep the notion of  cause and effect in order, time travelers may also face – or already have faced – other challenges from physics, even classical physics.

One issue you have to consider during time travel, and which science fiction writers usually prefer to ignore for convenience sake, is the question of arrival at the specified time destination and what would happen to us there.

It is usually assumed, with no good reason, that if someone is travelling through time, he or she will land in the same place, but at a different time – past or future. But this is where we hit a snag: the Earth rotates around the sun at a speed of 110,000 kph, and the Solar System itself is moving in its trajectory around the galaxy at a speed of 750,000 kph. If we time-travel for even a few seconds and stay in the same coordinates of space, we will probably find ourselves floating in outer space and perhaps even manage a quick glance around before we die. Our time machine will have to take into account this movement of the heavenly bodies and place us at exactly the right spot in space.

This alone may be resolved, since time travel, in any case, takes place between two points in the four-dimensional space-time continuum. According to the theory of general relativity, the theoretical foundation for time travel, space and time are a single physical entity, known as space-time. This entity can be bent and distorted – in fact gravity itself is an external manifestation of space-time distortion.

The Time Lord ,Doctor Who explains what “time” is exactly (Doctor Who, Season 3, Chapter 10: Blink).

Time travel would be possible if we could create a closed space-time loop, or if we could go from one point to another through a shortcut called a “Wormhole”. This would, in any case, not be just moving from one point in time to another, but would also include moving through space. Thus, from the outset, the journey is not only in time, but necessarily includes a destination point in space, which we will need to pre-program on our machine, of course .

In practice, the situation is more complicated – especially if we want to go into the distant past or distant future. The speed of the celestial bodies, and even the Earth’s shape and the structure of the continents, the seas, and mountains on the face of the Earth, change over the years. And because even a tiny deviation in our knowledge of the past can land us in the core of the Earth, in outer space or somewhere else that immediately reduces life expectancy to zero – time travel becomes a Russian roulette.

 How to travel in time and stay alive

 Let’s assume we coped with this problem and managed to get to the exact point in space-time that can sustain life. Careful – we’re not there yet; we still have to deal with momentum.

Momentum is a conserved quantity, which basically represents the potential of a body to continue moving at the speed and direction in which it is already travelling. If we were to jump out of a moving car (heaven forbid!), conservation of momentum is what would cause us to roll on the ground and probably get injured (in the best-case scenario). And so, if we leap in time – say, a month back – and land at the exact same point on Earth – we would discover, much to our dismay, that even if we started motionless in relation to the ground, now the ground underneath us is moving quickly at one angle or another towards us . Thus, even if we were lucky enough not to crash immediately on impact, we’re likely to hit some obstacle. And if by some miracle we were to survive, we would quickly find ourselves burning up in the atmosphere or gasping for air in space – because we have far exceeded the escape velocity from Earth.

A possible solution to this problem is to plan our landing point ahead, so that the ground speed will be equal in size and direction to our exit speed, but this places many constraints on our journey. We could always leap into space, where there are hardly any moving objects to be bumped into, and only then land again at our point of destination on Earth.

Having said all that, this problem arises chiefly when we assume that time hopping is immediate – that we disappear from one point in time and immediately appear in another, without losing mass, energy, or momentum. But since a “realistic” journey in time is not instantaneous, rather it involves travelling along space-time, it is no different from other types of journeys. This being the case, we can hope that we could adjust our speed to the desired value and direction prior to landing, just like a spacecraft slowing down before landing on a planet.

We should also keep in mind that thankfully, we will have access to a powerful technology that would enable us to cope with such problems: Time-travel technology itself. For example, we might decide to send thousands of tiny probes ahead of us, each to a slightly different point in space-time. Some of them, maybe even most, will be destroyed for one of the reasons already mentioned. The others will wait patiently until the present and then feed their programmed coordinates into the time machine. Thus by definition, the destination entered will be safe for us, except, perhaps for the annoying probe shower hitting the travellers. For the travellers themselves, the entire process will be immediate.

Time Travelling Grammar

Finally, we come to the question: How do you actually talk about time travel? The three tenses – past, present, and future – are insufficient to discuss a future event that happened some time in the past with someone who is in the present, which is another’s past and yet another’s future. And what is the correct grammatical tense to use when we talk about an alternative future that would have been created after we killed our grandfather? Or how do we express the future-past tense (or past-future, or past-future-past?), when we get stuck in a time loop where what will happen leads to what had already taken place, and so on? And of course the biggest question that Hebrew editors and translators have faced for years – is there really such a thing as present continuous?

It’s complicated.

Arguing about tenses and a time machine, The Big Bang Theory, Season 8, Episode 5, 2014

In his book, The Restaurant at the End of the Universe, science fiction writer Douglas Adams suggests to his readers to consult (by Dr. Dan Streetmentioner) Time Traveler's Handbook of 1001 Tense Formations (by Dr. Dan Streetmentioner) to find the answers to these questions. That’s all very well, but, Adams tells us, “most readers get as far as the Future Semi-Conditionally Modified Subinverted Plagal Past Subjunctive Intentional before giving up; and in fact in later editions of the book all pages beyond this point have been left blank to save on printing costs.”

If, despite all of the above, you’re still intent on travelling back to Mount Sinai or the Apollo 11 moon landing – let us then wish you bon voyage, and please give our regards to Neil Armstrong!

Screen Rant

13 back to the future plot holes & time travel paradoxes (& which ones have been fixed).

Back to the Future is full of time travel plot holes and paradoxes, and while these 13 are the most apparent, some of them have actually been fixed.

Like all time travel stories, Back to the Future has several plot holes and time travel paradoxes, and these 13 are the biggest ones. Due to the nature of time travel and constantly changing timelines, Back to the Future has to keep track of a lot of small details, but some manage to slip through the cracks. Luckily, the Back to the Future trilogy has fixed some of these plot holes and paradoxes, although others are still in existence.

The Back to the Future trilogy mainly takes place in four different time periods. 1985 is the present across all three movies, while Marty and Doc travel to 1955, 2015, and 1885 throughout the trilogy. Due to the constant traversal between the past, present, and future, all kinds of alternate Back to the Future timelines are opened throughout the franchise, leading to some serious paradoxes. So, here are the 13 biggest plot holes and paradoxes from the Back to the Future series as well as which ones were fixed.

13 George & Lorraine Should Recognize Their Son As Marty McFly

One of the biggest plot holes in the Back to the Future series is that George and Lorraine McFly should have definitely realized that their son looked exactly like Calvin Klein in 1955. Although they didn't know Marty's alternate 1955 persona for long, he played a big role in both of their lives. Because of this, it is strange that neither of them ever mentioned how familiar Marty looked, although it makes sense that they wouldn't leap to the idea that Marty was a time traveler.

Related: Why Back To The Future Has Aged So Well (When So Many Scenes Haven't)

12 Marty McFly Shouldn't Have A Hairdryer In 1955

One plot hole that frequently gets brought up is that Marty McFly shouldn't have a hairdryer in 1955, although this has actually been fixed. In the first film, Marty puts on a sci-fi costume in an attempt to scare the 1955 version of George McFly. However, part of his costume is a handheld hairdryer, which shouldn't exist in 1955. The creators of Back to the Future answered this question , explaining that a deleted scene featured Marty pulling a hairdryer out of a suitcase that 1985 Doc put in the DeLorean.

11 The Clock Tower Should Work After Marty Gets Back To 1985

The broken clock tower is one of the most important elements of Back to the Future , with it ceasing to work after being struck by lightning in 1955. However, Marty and Doc change history by rerouting the lightning to Marty's DeLorean, sending him back to the future. Because of this, the clock tower should work when it is seen in Back to the Future Part II , although it is still broken for some unexplained reason.

10 Old Biff's Fate Was Never Revealed

A massive plot hole from Back to the Future Part II is that Old Biff's fate was never revealed, with him simply walking away after returning to 2015. This is another detail that is actually explained by a deleted scene. Since Old Biff gave Young Biff the sports almanac, this changed the future, meaning that Old Biff would cease to exist. Because of this, a deleted scene showed Old Biff falling over and fading away, although it was cut from the final film.

9 Marty Didn't Have To Steal The Almanac On November 12, 1955

The second half of Back to the Future Part II is pretty tense, with it following Marty as he tries to steal the sports almanac from Biff on November 12, 1955. Marty chooses this date because it is when Old Biff gives Young Biff the almanac, but choosing November 12, 1955, adds a lot of unneeded stress to the situation. Biff didn't use the almanac until several years after he got it, meaning Marty could have simply traveled to a time when it was unoccupied and taken it that way.

8 Old Biff Went Back To The Wrong 2015

Another paradox surrounding Old Biff is that he returned to his original 2015 timeline after giving Young Biff the sports almanac, but this actually shouldn't happen. Since Old Biff changed the future, 2015 should be completely different, but it looks the same when Biff returns. The creators have also explained this, saying that the timeline would either change around Old Biff or that the neighborhood he was in simply looked the same despite the timeline changes.

7 Old Marty & Old Jennifer Shouldn't Exist In 2015

Back to the Future Part II introduce Old Marty and Old Jennifer, introducing one of the series' biggest paradoxes. 1985 Marty and Jennifer meet 2015 Marty and Jennifer when traveling to 2015, but their older selves are from a timeline where they didn't travel to 2015. Since 1985 Marty and Jennifer did travel to 2015, this version of their older selves shouldn't exist and they should have been replaced by an Old Marty and Old Jennifer that did travel to the future when they were younger.

6 The DeLorean Time Travels Without Going 88MPH In Back To The Future 2

In Back to the Future II , the DeLorean time machine spins around before time traveling away, breaking the rule that DeLoreans must go 88 miles per hour to time travel. As it turns out, though, this problem has been fixed. Bob Gale and Robert Zemeckis have explained that the DeLorean doesn't necessarily have to be moving forward 88 miles per hour, and since it was spinning at that speed, it was able to time travel.

5 There Are Two DeLoreans In 1885

One of the franchise's most famous plot holes occurs in Back to the Future Part III 's 1885 timeline. During the film, there are actually two DeLoreans in the same location: Marty's DeLorean as well as the DeLorean Doc used to travel to 1885. Although they are both broken, Doc and Marty could have combined parts from the two cars in order to make one that worked, solving the film's problems much faster.

4 Doc Brown Should Know About Einstein In Back To The Future 2

In Back to the Future Part III , 1955 Doc smirks when he learns that his future dog is named Einstein. However, Einstein was in the tape that Marty showed 1955 Doc in the first film, meaning that he should have already known this. This plot hole has also been fixed, with the creators explaining that Doc simply didn't see or remember this part of the tape.

3 Doc's Tombstone Shouldn't Exist In Back To The Future 3

In Back to the Future Part III , Marty sees Doc's tombstone from 1885, opening up another paradox. Since Marty went back in time to prevent Doc's murder, the tombstone shouldn't exist, meaning that 1985 Marty would have never seen the tombstone and gone back to 1885 in the first place. This massive paradox ruins the movie's story, although the tombstone had to exist in order to serve as the story's inciting incident.

Related: Back To The Future: Doc's Death Plot Hole Explained

2 Clayton Ravine Should Have A Different Name If Clara Lives

In Back to the Future Part III 's main timeline, teacher Clara Clayton falls into the Shonash Ravine, causing it to be renamed Clayton Ravine. However, when Doc travels back in time, he falls in love with Clara and consequently prevents her from dying. When 1985 Marty sees Doc's grave, he is in the timeline where Clara lived, yet he still knows about the Clayton Ravine legend. Since Clara never died, Marty's knowledge of Clara's death shouldn't exist, and it should still be named Shonash Ravine. However, the franchise's creators have clarified that time travel doesn't affect memory, which is an odd but valid solution.

1 Doc Should Know That "Mad Dog" Tannen Is Going To Kill Him

The final major plot hole and paradox in the Back to the Future series occurs in the third movie, where Doc is killed by "Mad Dog" Tannen. The version of Doc that is killed comes from 1985, meaning that when Marty tells 1955 about Doc's death, 1985 Doc should know as well. Because of this 1985 Doc would be able to prevent "Mad Dog" Tannen from killing him, negating Marty's reason for traveling back to 1885 in the first place. While many of Back to the Future 's plot holes and paradoxes are confusing, this one is one of the most straightforward.

The paradox of time travel, answered!

Everyone has, at least, once in a lifetime thought of the idea of time travel. “What if I could go back in time and change the course of my life.” Time travel was made extremely popular with the release of the blockbuster movie, “back to the future.” Since then many science fiction movies have emerged with time travel as the center of their plot. Time travel has always been linked to science fiction and unachievable but Einstein's theory of relativity has opened new doors. It has shown us that time is relative and does not pass at the same speed for everyone. But still many scientists believe that time can be squashed or stretched, but it cannot run backward. Even though if we achieve time travel, another problem arises and that is, the right position. Nothing in the universe is stationary, as the moon revolves around the earth, the earth around the sun and the sun around the milky way, time travel isn't merely about moving through time but also distance. If you were to travel back in time 100 years to the exact same position you are in right now, the earth would not have arrived yet, it would still be millions of kilometres away. So not only would we need to figure out how to travel in time, but also where everything would be relative to that time period. However, if we resolve this vicious cycle of time travel then we have another problem knocking our doors and that is time paradoxes.

A paradox is a flaw in logic. It is a statement which contradicts itself, like the statement “nobody goes to x restaurant anymore as it is too crowded.” Time travel is filled with many paradoxes. Some of the most famous paradoxes are, “the grandfather paradox”, “the predestination paradox” and “the bootstrap paradox.” This post covers various types of paradoxes and their possible solutions. Therefore bear with me to dive into the mind melting theories and paradoxes of time.

The paradoxes of time:

Grandfather paradox: Grandfather paradox is by far the most famous time paradox. It says that if a person travels to a time before their grandfather had children and kills him making their own birth impossible. So how was he able to move back in time in the first place?

Bootstrap paradox: Bootstrap paradox says that if you send an object back in time, it would be stuck in an infinite time loop with no origin. For example, you are a child studying the works of Shakespeare at your school. You travel back in the time of Shakespeare with a copy of “Hamlet” and you hand over the book to Shakespeare who later publishes the book by his name. Then who actually authored the book? You read Hamlet approximately four centuries later the book was published and you hand over the book to Shakespeare back in time who copies the book and publishes it by his name, then where does the book actually come from? The book has no origin. It is stuck in an infinite time loop .

Predestination paradox: A predestination paradox arises when a person traveling back in time becomes part of past events and may even have caused the initial event that caused that person to travel back in time in the first place. For example a fire set up in a forest and you decide to investigate the source of the fire. You go back in time, but you accidentally knock over a box of kerosene which ignites the fire, the same fire that would inspire you, years later, to travel back in time.

Time paradoxes can be pretty mind bending, but we have many hypotheses which solve these paradoxes. The most straightforward solution is that you can’t go back in time, thus these paradoxes would never arise. But this theory easily dodges the paradox ignoring the fact that everything is possible (given time constraints to achieve it). Thus, what if time travel is possible as we never know what the future looks forward to. If it is possible, then what could be the solution of the paradoxes?

The multiverse theory: The multiverse theory states that when a choice is made (no matter how insignificant), a plane of existence is created for each outcome. For example, there's a universe where that person didn't eat the bat. There's a universe where you partook of breakfast two seconds later than you did today. There's a universe where the wind blew at 5.00000001 MPH instead of just five MPH. Every outcome exists in parallel universes.

Now back to the paradox. You can visualize the theory as a tree graph, where every node represents a choice and every branch is an outcome. Let's make a simple chart that represents the aliveness of your grandfather.

/ (Yes, you are born)

(Grandfather Alive?) ⟶

\ (No, you aren't born)

Hence, you are in the top branch. Let's say you travel back to the first node and kill your grandfather. Do you break time and cease to exist? No!

You simply make a change in another universe where the change already existed. When you go back in time to kill your grandfather, you're not going back to your own history, but a copy of your history, and everything you do in this version of your history will affect the alternate future of that universe, not your own. It is the universe where your grandfather was dead, and it would not affect the timeline you are living in.

Information takes time to travel: Changes you make can never catch up to the present. Those changes can't move through time instantly. It's subject to a speed limit. We all are time travelers. We all are moving in the future every second. The speed of time is.. one second per second so if you go back five years in time and make a change, then for it to reflect, it should take another five years.

Imagine that a timeline is like a giant history book where a time traveler is erasing the past chapters, but the author who is writing the book is not aware of the changes. With this another possibility arises: the changes you make will just remain there and never ever affect the future. It's because the information is lost between dimensions and time. Maybe that's what is deja vu. Your future self, visiting the past and doing a particular task which is stuck in the dimension, and you also do the same task which leads you to think that you have already done that!

Time loop: If you go back in time and kill your grandfather, you will never be born thus you can't move back in time, so your grandfather isn't killed, thus you are born, so you go back in time and kill your grandfather, and so on… It's two entangled events happening together. This is possible at quantum states, and subatomic particles do this almost always which is called superposition and this is responsible for fusion in the sun's core, double slit experiment, etc. So if the world was to exist in the superposition state, then your grandfather is both dead and alive forming a closed time loop. But could a closed time loop actually exist, according to the laws of physics? We don't know.

Even though all these solutions are completely hypothetical as no individual has visited the past (as far as we know) but they provide satisfactory answers nonetheless this could also be a possibility that we are knocking the wrong doors. Whatever the true answer may be, we are miles away to achieve it. We have to attain tons of knowledge of time itself, leave alone time travel. Even though time travel could appear impossible to us, this field is growing and maybe one day quantum physics could answer the time paradoxes and maybe our dreams of time travel could be true, one fine day.

Recent Posts

What and where are extra dimensions?

Why parallel universe exists

Why is there anything instead of nothing?

• The Magazine
• Stay Curious
• The Sciences
• Environment
• Planet Earth

How Does Multiverse Theory Relate to Time Travel?

The theoretical physics and paradoxes of time travel often brush up against multiverse theory and the idea of alternate universes..

While many sci-fi writers have wondered if time travel into the past can be allowed as long as you slip into an alternate universe, you should be warned: Nature doesn’t like a cheater.

Time travel into the past is a real pain in the neck. For one, as far as we can tell it’s forbidden in our universe. You can travel into the future as slowly or as quickly as you like, but you can’t stop time or go in reverse.

Is It Possible To Travel Back in Time?

Technically speaking, physicists are not exactly sure why time travel is forbidden. We don’t have a single law of physics that clearly rejects the possibility.

But every time we try to concoct a time travel concept — like wormholes or twisting paths around infinitely long cylinders — we find some reason that it’s not allowed. Wormholes, it turns out, require negative mass to stabilize themselves, and infinitely long cylinders are rather hard to come by. 

Read More: Why Do Humans Perceive Time The Way We Do?

The Trouble With Changing the Past

Even within any glimmer of a possibility that time travel might be possible, traveling into our own pasts opens up all sorts of noxious paradoxes, as plenty of films and comic book storylines have demonstrated.

Beyond the fact that the past is over and done with, we must reckon with the reality that what happened in the past created the present that we experience.

To put the paradox succinctly: If you change the past, you change the present, but the present already exists.

The Grandfather Paradox

One of the most common examples of this head-scratching scenario is known as the grandfather paradox.

Suppose you travel back in time and kill your own grandfather before he met your grandmother (no need to dissect your motivations behind this hypothetical act here). That means one of your parents was never born, which means you never exist. But if you never existed, how did you go back in time to kill your grandfather in the first place?

Read More: What Is the Grandfather Paradox of Time Travel?

Are There Multiple Universes?

To address this dilemma, we could invoke the idea of parallel or alternate universes, a concept under the general umbrella of the multiverse.

Perhaps when you go back and kill your grandfather, you create a new universe where you don’t exist. You can then return to your original universe where everything is hunky-dory (or, at least you’re still alive). Essentially, you only monkeyed around with some other, disconnected cosmos.

There are, in fact, two places in physics where the multiverse concept pops up. Unfortunately for any wannabe time travelers, neither of them allows for this type of parallel-futures hopping. 

Splitting-Off Worlds in Quantum Mechanics

The first place where alternate universe theories have gained traction is in quantum mechanics.

Subatomic processes are inherently random. When we run an experiment we never know what result we’re going to get. One explanation for this is that all experimental results happen, just in separate universes.

If someone shoves an electron through a strong magnetic field, for example, and it has a fifty-fifty chance of deflecting either up or down, then every time they run an electron through the device, they get two universes: one where the electron went up, and one where the electron went down.

As the theory goes, this kind of splitting isn’t just limited to arcane physics experiments. It happens with every single quantum interaction throughout the cosmos, which is a lot of interactions, thus generating countless parallel worlds.

In this view, there is a universe out there that already exists where your grandfather is dead and you were never born. But you could never see it.

Read More: Advanced Quantum Material Curves the Fabric of Space

No Travel Between Worlds in Quantum Mechanics

Even if the theory above is actually happening, the rules of quantum physics are absolutely clear: There is no travel or communication between these universes.

Granted, “universe” is probably the wrong word here. That’s because the splitting is really separate partitions of a single quantum wave function. And that separation is really permanent.

If there was any kind of connection allowed, then the many branching possibilities of quantum processes simply wouldn’t happen.

Bubble Universe Theory

The second place where the multiverse pops up in physics is in studies related to the extremely early universe.

From this lens, our cosmos could be just one of many — like a bubble in an infinite foam of universes. These universes too could contain all the many interesting and varied possibilities that make life so fun.

In this (extremely hypothetical) view, there are many universes out there that contain alternate realities from the one you know. There’s one where your grandfather died. There’s another where no stars ever formed. And there’s a universe where all your matter is replaced with antimatter, and so on.

Because all these bubble universes exist in the same expanding foam that is reality, those alternate universes really are out there , an unimaginable but still-finite distance away.

Time Still Marches Forward in the Multiverse

But even in this foam-like collection of bubble universes, time still does its thing. That is, it moves forward, always. 

Even if you could concoct some scheme to travel among the alternate universes, those universes still share the same spacetime fabric, marching forward along with us. Thus they all appear to forbid time travel, as the theory goes.

They also, in theory, share the reality that nature doesn’t like a cheater. So, if you’re theoretically going to visit a universe where you never existed, maybe nix the time travel method.

Read More: Scientists Attempt to Map the Multiverse

• subatomic particles

Already a subscriber?

Register or Log In

Keep reading for as low as $1.99!

Sign up for our weekly science updates.

Save up to 40% off the cover price when you subscribe to Discover magazine.

Doctor Who Is Finally Addressing A Classic Time Travel Trope

Is Doctor Who changing the laws of time? Again?

The oldest science fiction series — that still produces new episodes — is easily Doctor Who . In 1963, the inventive time travel series debuted on the BBC, and on Friday, May 10 , this venerable show relaunches anew with a brand-new set of adventures featuring Ncuti Gatwa as the 15th Doctor. But this Doctor Who is coming back as “Season 1,” meaning that although the show is honoring its long and wibbly-wobbly continuity, this new season feels like a bit of a back-to-basics season, complete with one very old time travel trope.

In the new trailer, Doctor Who references a time travel concept nearly as old as science fiction itself — the butterfly effect. Here’s how this very specific reference pulls from Ray Bradbury, and what this brand of time travel could mean for the future of Who canon.

Doctor Who Season 1 (2024) trailer

The latest full-length trailer for the new 2024 season of Doctor Who features a joyful version of David Bowie’s classic track “Changes,” as we see the Doctor and Ruby (Millie Gibson) travel through various historical periods; from the 1960s, to the 1800s regency era, to the prehistoric time of the dinosaurs in the Mesozoic era. And it’s in those dino-days that Who drops a very pointed Ray Bradbury reference, that might give a clue to the direction of the entire season.

The timey wimey butterfly effect

As the Doctor and Ruby stand outside the TARDIS near a large sauropod (probably a Brachiosaurus), Ruby says, “What if I change history by stepping on a butterfly?” The Doctor responds, “That’s not gonna happen is it?” But then, a butterfly is stepped on, and Ruby becomes a reptilian humanoid — similar in appearance to Who creatures known as the Silurians.

The idea of a time traveler stepping on a butterfly in dinosaur times and thus altering the present comes from the 1952 Ray Bradbury short story “A Sound of Thunder.” In that story, members of a time-traveling safari step off their designated path, step on a butterfly and return to a present that they don’t recognize. The Back to the Future version of this was the Sports Almanac, and the trope exists in tons of sci-fi stories in all sorts of mediums.

But, Doctor Who has never really gone all-in on the butterfly effect. In previous eras, including the reign of the 10th Doctor (David Tennant) we were told certain points in time were “fixed” and thus, unchangeable, while other times were in flux. In “The Fires of Pompeii,” the Doctor was unable to prevent the eruption of the volcano but became part of a predestination paradox that made him and Donna part of history. Conversely, in “The Waters of Mars,” the Doctor tried to change a fixed point in time by rescuing astronauts from Bowie Base One (David Bowie reference again!) and found time itself rebelling against what he’d tried to change.

Of course, Doctor Who would frequently lampshade the butterfly effect, like in the 12th Doctor (Peter Capaldi) episode, “Thin Ice,” in which companion Bill Potts (Pearl Mackie) wonders if she would change the future by stepping on a butterfly, only for the Doctor to joke, “That’s what happened to your friend Pete.” (There is no Pete.)

Is Doctor Who changing time travel canon?

The Doctor and Ruby surrounded by dinosaurs...and butterflies.

Throughout its 61 years, Doctor Who has played a bit fast and loose with the butterfly effect relative to cascading changes in the timeline. Interestingly, although the classic era (pre-2005) finds the various Doctors showing up in historical eras, or traveling to the far future, the number of classic Who episodes that are actually focused on time travel paradoxes are much smaller than you think. While “The Day of the Daleks” (1972) and “City of Death” (1979) both deal with bootstraps paradoxes, it really wasn’t until the modern version of the show that the rules of Who time travel started to get codified.

However, this trailer seems to suggest that the newly launched “Season 1” of Doctor Who may go back to basics and put everything on the line when it comes to the domino effects of even tiny changes in history. Later in the trailer, the Doctor warns “...the whole world could slide into the pit,” and we see various timelines in which it appears that history, or at least the present, has been changed on a massive scale. The Doctor states clearly, “This is what we’re trying to stop.”

In this way, it seems that the new season of Doctor Who is making the role of the Doctor more than just the defender of Earth. Since he’s still the Last of the Time Lords, it appears that this time, he’ll be the guardian of history as we know it.

Doctor Who Season 1 (2024) hits Disney+ on May 10.

• Science Fiction

Using 'time travel' to think about technology from the perspective of future generations

The world approaches an environmental tipping point, and our decisions now regarding energy, resources, and the environment will have profound consequences for the future. Despite this, most sustainable thought tends to be limited to the viewpoint of current generations.

In a study published in Technological Forecasting and Social Change , researchers from Osaka University have revealed that adopting the perspective of "imaginary future generations" (IFGs) can yield fascinating insights into long-term social and technological trends.

The researchers organized a series of four workshops at Osaka University, with participants drawn from the faculty and student body of the Graduate School of Engineering. The workshops discussed the state of future society and manufacturing in general, and also looked at one technology in particular: hydrothermally produced porous glass. During the workshops, the participants were asked to think about this technology from the perspective of IFGs, to imagine how this technology might be adopted in the future and to assess its future potentiality.

"We chose hydrothermally produced porous glass for the case study because of the generational trade-offs involved," says lead author of the study Keishiro Hara. "Porous glass is incredibly useful as either a filter for removing impurities or an insulator for buildings. Also, it can be recycled into new porous glass more or less indefinitely. The problem is that making it takes a lot of energy -- both to pulverize waste glass and to heat water to very high temperatures. There's a striking trade-off between costs now and gains in the future."

In the workshops, the participants first looked at issues involving society and manufacturing from the perspective of the present and were then asked to imagine themselves in the shoes of their counterparts in 2040.

"The future the participants imagined was quite different from the future as seen from the perspective of the current generation," explains Toshihiro Tanaka, senior author. "Most groups described a future in which sustainability has become a central concern for society. Meanwhile, advances in renewal energy mean that energy is abundant, as are resources, as frontiers such as the moon and deep ocean are opened to exploration. In this context, hydrothermally produced porous glass comes into its own as a sustainable way to recycle glass, and the energy needed to produce it is readily available."

The participants were surveyed between workshops and asked to rank indicators related to the future potentiality of the technology. Interestingly, these rankings looked quite different after the workshops in which the participants were asked to take on the perspective of "imaginary future generations."

"We noticed that when the "imaginary future generations" method, which has been proven to be effective in facilitating long-term thinking, was adopted, participants perceived the feasibility of this technology differently, and their adoption scenarios changed accordingly," says Hara.

The study suggests that the simple act of putting ourselves in the position of future generations may provide new perspectives on issues of sustainability and technology, helping us to rethink our priorities and set new directions for research and development.

• Energy Technology
• Energy and Resources
• Engineering and Construction
• Educational Technology
• Information Technology
• Computer Science
• Environmental Policies
• STEM Education
• Energy Issues
• Emerging technologies
• Engineering
• Nanomedicine
• Scientific method
• Liquid nitrogen economy
• Earth science

Story Source:

Materials provided by Osaka University . Note: Content may be edited for style and length.

Journal Reference :

• Keishiro Hara, Iori Miura, Masanori Suzuki, Toshihiro Tanaka. Assessing future potentiality of technologies from the perspective of “imaginary future generations” – A case study of hydrothermal technology . Technological Forecasting and Social Change , 2024; 202: 123289 DOI: 10.1016/j.techfore.2024.123289

Cite This Page :

Explore More

• Illuminating Oxygen's Journey in the Brain
• DNA Study IDs Descendants of George Washington
• Heart Disease Risk: More Than One Drink a Day
• Unlocking Supernova Stardust Secrets
• Why Do Some Memories Become Longterm?
• Cell Division Quality Control 'Stopwatch'
• What Controls Sun's Differential Rotation?
• Robot, Can You Say 'Cheese'?
• Researchers Turn Back the Clock On Cancer Cells
• Making Long-Term Memories: Nerve-Cell Damage

Trending Topics

Strange & offbeat.

This Is the Way the World Ends (According to Novelists)

A new kind of disaster fiction is serving as scenario planning for real global crises. Call it the apocalyptic systems thriller.

Credit... Anuj Shrestha

Supported by

• Share full article

By Hari Kunzru

Hari Kunzru’s next novel, “Blue Ruin,” will be published in May.

• March 29, 2024

In the opening chapters of “2054,” a new thriller co-written by Elliot Ackerman and Adm. James Stavridis, the action shifts every page or two — from a private jet, to the White House, to a Ritz-Carlton hotel, to Capitol Hill and elsewhere. The short sections are titled with locations and time stamps (including time zones) to signify global scope and high stakes. We understand at once that this is a story too big to be told from one perspective.

It’s a familiar technique; in movies, such titles are often given extra technical sheen with a flashing cursor that prints them out across the screen, a skeuomorphic legacy of teletype and early command-line interfaces. The implication is that diverse situations are being monitored in some way, logged and recorded by a technically proficient authority that sees them as part of a coherent whole.

“2054” is a sequel to “2034,” “a novel of the next world war” that meticulously laid out a sequence of events starting with a naval confrontation in the South China Sea and ending in nuclear conflict. Twenty years afterward, Sandy Chowdhury, a character in the first novel, asks the pilot of his Gulfstream to divert so he can view the reconstruction of Galveston, Texas, which was leveled by a Chinese warhead. The physical consequences of the devastation are being repaired, but the social and political divisions remain.

The geopolitical epic is at least as old as “War and Peace,” but there’s a particular kind of novel that came into its own with globalization, taking on new life in recent years. Call it the apocalyptic systems thriller, or, because abbreviations and acronyms are crucial to its aesthetic, the A.S.T.

Multi-stranded, terse, often anchored in character just enough to drive the action forward, these books invite us to take an elevated, panoramic view of events that extend too far in space and time to be grasped by a single narrative consciousness. Conflict, climate change, pandemics and natural disasters offer ways to contemplate our interconnection and interdependence. At its best, this kind of fiction can induce a kind of sublime awe at the complexity of the global networks in which we’re enmeshed: A butterfly flaps its wings in Seoul and the Dow crashes; a hacker steals a password and war breaks out.

The currency of the A.S.T. is plausibility. It can be counterfactual, but never fantastical. It differs from other kinds of thrillers in its willingness to indulge in essayistic digressions about technology or policy. In some cases, the story may even take second place to these ideas, a mere vehicle for the delivery of an info-payload. In this, the A.S.T. is essentially a subgenre of SF, or at least the kind of science fiction that prioritizes world-building over other kinds of narrative pleasure. Indeed, many A.S.T.s, like “2034” and “2054,” are near-future tales, extrapolating from the present to a carefully imagined next five minutes, designed to elicit a little spark of recognition, the feeling of being shown a possible path from “here” to a utopian or dystopian “there.”

“The End of October,” Lawrence Wright’s eerily prescient novel about a global influenza pandemic, was written before Covid and published in the early days of the 2020 lockdown. Like Steven Soderbergh’s 2011 pandemic drama, “Contagion,” which experienced a boom in streaming in early 2020, Wright’s book generated a lot of affect, at least in that moment, from its power of extrapolation. We were in the early days of something; we didn’t know what. Here were narratives that showed what we could be facing if the pandemic intensified to the point of mass death and social breakdown. Naturally, we could not look away.

To succeed, this kind of story has to feel true. Even as the A.S.T. indulges in thriller tropes, the chases and explosions must be anchored by a kind of epistemological authority. We must believe that what we’re reading is something more than a product of the writer’s fevered imagination. Wright, who won a Pulitzer for “The Looming Tower,” his account of 9/11, derives his authority from extensive and rigorous research. In the acknowledgments (always an important part of an A.S.T.) he credits virologists and public health officials, as well as a number of submariners, who helped with an extended sequence that takes place aboard one of the U.S. Navy’s long-range underwater vessels.

Ackerman and Stavridis derive their authority from their military experience. Ackerman served five tours of duty in Iraq and Afghanistan as a Marine. Stavridis retired as a four-star admiral and served as NATO’s supreme allied commander for Europe. When they’re describing the buildup to war in “2034,” their procedural knowledge gives their story a verisimilitude that elevates its workmanlike telling. As Navy men, they are not above a little light trolling of the other branches of the armed forces: How else are we to understand a novel about World War III without a single mention (as far as I’m able to tell) of the Army?

The interwoven character arcs of “2034” convey what really seems to interest the authors — a set of warnings and predictions aimed at their peers in what used to be called the military-industrial complex. The ladder of escalation begins with a Chinese demonstration of decisive cyberwar superiority, a capability gap that leads to disaster. The vulnerability of global communications systems is a preoccupation, as is the rise of India as a military power. The important theaters of war are naval ones — the South China Sea and the Strait of Hormuz. So maybe that’s where military spending ought to be directed? Just a thought from the admiral.

The sequel, however, turns its attention to biotechnology and American domestic partisanship. The president dies suddenly from a mysterious growth on his heart, raising fears of a new kind of bioweapon based on “remote gene editing” technology. His death leads to civil unrest, and takes a motley crew of characters, including Chowdhury, a genetics researcher and a seductive diving instructor, through an eccentric riff on “Heart of Darkness.” They head upriver to find a reclusive figure — not Kurtz, but a centenarian Ray Kurzweil, the ebullient (and real) Silicon Valley futurist and promoter of transhumanism. The future of humanity will be decided in the lab, the authors caution, and we are sorely unprepared.

Experts on military matters, Ackerman and Stavridis are much less secure on scientific terrain. Their notion of the world-changing “singularity,” drawn from Kurzweil, is little more than metaphysical hand-waving, and their descriptions of biotechnology lack substance, which drains their tale of the most important quality of any A.S.T.: procedural plausibility.

The most celebrated example of the A.S.T. in recent years — and the book that, in its breadth of speculation, sets the standard for the genre — is Kim Stanley Robinson’s “The Ministry for the Future,” which attempts the enormous task of imagining a coordinated global response to another grave threat to humanity, climate change. From its terrifying opening set piece, which portrays a lethal heat event in north India, it expands across the globe and through decades, laying out strategies for alleviating the crisis. Along the way, the story is studded by capsule essays on a range of technical topics, from the Gini Coefficient and tax policy to carbon sequestration and the Jevons Paradox .

This is fiction as simulation, running versions of events, trying to imagine how things might be if the pieces on the board were arranged in a certain way, rather than playing the other games novelists play, imagining what could never be or simply never was. One ancestor of this kind of work is the 19th-century social novel: Think of Zola’s meticulously researched Rougon-Macquart sequence, a 20-book panorama of life under the French Second Empire. A more recent predecessor is the near-future cyberpunk narrative exemplified by Neal Stephenson and William Gibson. Still another would be the kind of techno-thriller pioneered by Tom Clancy and Michael Crichton, particularly the spate of novels published late in the Cold War, including Clancy’s “Red Storm Rising” and Gen. Sir John Hackett’s “The Third World War,” which imagined hostilities between NATO and the Warsaw Pact. But there’s a different tradition behind the A.S.T., one outside conventional literary history — corporate scenario planning.

During the Cold War, one think tank was central to the business of imagining the American future. The RAND Corporation had been spun out of the Department of War to produce policy proposals for the U.S. government. It used game theory to model nuclear escalation and collected what would now be called “big data” (in retrospect, woefully small) to guide the conduct of the Vietnam War. A RAND futurist called Herman Kahn began to supplement mathematical models with what he termed “scenarios,” war-game narratives illustrating “rungs of the escalation ladder” of nuclear conflict, from “ostensible crisis” through “justifiable counterforce attack” to “spasm or insensate war.” The storytelling element was a daring innovation in an organizational culture that valued quantitative analysis.

In the early 1970s, the oil company Royal Dutch Shell hired the Frenchman Pierre Wack to run its strategic planning department. A disciple of Kahn, Wack was also an early corporate adopter of “mindfulness” and a student of the Caucasian mystic George Gurdjieff. Wack made scenario planning a kind of structured science fiction, producing not a single forecast but several competing images of the future. Participants were encouraged to step away from the “official future” — whatever the orthodoxy might be in their organization.

Wack’s scenario planning was credited with helping Shell weather the oil shocks of the ’70s, and this style of “possible future” storytelling gradually spread beyond the company, finding fertile ground in the emerging Bay Area tech scene. By the 1990s, the founder of the Whole Earth Catalog, Stewart Brand, was a partner in a scenario planning consultancy. Wired magazine ran a “scenarios” issue in 1995, detailing possible futures imagined by writers of speculative fiction like Stephenson, Bruce Sterling and Douglas Coupland, completing the fusion of scenario planning with more traditional literary pursuits.

So, while the A.S.T. is a form of entertainment, it’s also meant to enlighten the planners and decision makers who might grab a hardcover off the shelf at an airport bookstore. Bill Gates and Barack Obama have recommended “The Ministry for the Future.” Robinson spoke at the 2022 World Economic Forum in Davos. Stavridis and Ackerman have spoken at think tanks like the American Enterprise Institute, the Atlantic Council and the Carnegie Council for Ethics in International Affairs. As fiction for the Davos set, the A.S.T. is a tool for both forecasting and navigating the troubles to come.

This dual purpose is made particularly clear in “AI 2041,” a recent book by Kai-Fu Lee, the former president of Google China, and Chen Qiufan, one of China’s most celebrated SF writers. In it, Chen writes 10 short stories, each illustrating a scenario that Lee wants to discuss. Lee introduces the stories and writes an extensive commentary after each one, detailing the technological possibilities and social issues that it raises. Chen’s fictions are rather swamped by Lee’s context, which makes them into something like moral fables, mere illustrations of his points.

Why is the A.S.T. so salient right now? What itch is it scratching? One of the most astute thinkers about the emergent networked future is the design theorist Benjamin Bratton. In “The Revenge of the Real,” a work of nonfiction published in 2021, he proposes a “politics for a post-pandemic world,” suggesting that Covid has trained us to see ourselves in an “epidemiological” way: Like it or not, we are, inescapably, a population as well as individuals. We have undergone a kind of crash course in systems thinking that will, Bratton hopes, force us to approach our problems at global scale. “It is necessary,” he writes, “for a society to be able to sense, model and act back upon itself, and it is necessary for it to plan and provide for the care of its people.”

The aesthetic of the A.S.T., with its flaunting of globalization, its pleasure in technical advances and its refusal of the “single window” into its stories, does have a utopian dimension — the imagination of what Bratton calls “planetary competency.” The message is one of resilience, of human beings acting in concert, muddling through problems in the hope of navigating what Pierre Wack called “the rapids” of the near future, into calmer waters beyond.

Explore More in Books

Want to know about the best books to read and the latest news start here..

James McBride’s novel sold a million copies, and he isn’t sure how he feels about that, as he considers the critical and commercial success  of “The Heaven & Earth Grocery Store.”

How did gender become a scary word? Judith Butler, the theorist who got us talking about the subject , has answers.

You never know what’s going to go wrong in these graphic novels, where Circus tigers, giant spiders, shifting borders and motherhood all threaten to end life as we know it .

When the author Tommy Orange received an impassioned email from a teacher in the Bronx, he dropped everything to visit the students  who inspired it.

Do you want to be a better reader?   Here’s some helpful advice to show you how to get the most out of your literary endeavor .

Each week, top authors and critics join the Book Review’s podcast to talk about the latest news in the literary world. Listen here .

Advertisement

Frequently Asked Questions About Time Travel Streaming: Watch & Stream Online via HBO Max

By Dipesh Ramdasani

Ever wondered what would happen if you stumbled into a time travel conundrum over pints with your mates? That’s the quirky premise of the 2009 British sci-fi comedy Frequently Asked Questions About Time Travel.

Here’s how you can watch and stream Frequently Asked Questions About Time Travel via streaming services such as HBO Max.

Is Frequently Asked Questions About Time Travel available to watch via streaming?

Yes, Frequently Asked Questions About Time Travel is available to watch via streaming on HBO Max.

The film follows Ray and Pete, two friends whose passion for science fiction might just land them on a real-life adventure. Along with their more skeptical buddy Toby, they encounter a mysterious woman named Cassie at their local pub. Cassie, it turns out, is from the future, and she throws their world into chaos by revealing a hidden truth: time travel exists, and it’s way messier than they imagined.

Instead of futuristic spaceships and dystopian futures, the film explores the mind-bending paradoxes of time travel, leaving you chuckling and pondering the consequences of messing with the past.

Helmed by Gareth Carrivick, the main cast of the movie includes Chris O’Dowd as Ray, Dean Lennox Kelly as Pete, Marc Wootton as Toby, and Anna Faris as Cassie.

Watch Frequently Asked Questions About Time Travel streaming via HBO Max

Frequently Asked Questions About Time Travel i s available to watch on HBO Max. HBO Max is a streaming platform offering a vast library of TV shows, movies, and exclusive content. Backed by the powerhouse Warner Bros., it’s your one-stop shop for everything from blockbuster hits to binge-worthy series.

You can watch via Max, formerly known as HBO Max, by following these steps:

• Go to HBOMax.com/subscribe
• Click ‘Sign Up Now’
• $9.99 per month or $99.99 per year (with ads)
• $15.99 per month or $149.99 per year (ad-free)
• $19.99 per month or $199.99 per year (ultimate ad-free)
• Enter your personal information and password
• Select ‘Create Account’

Max With Ads provides the service’s streaming library at a Full HD resolution, allowing users to stream on up to two supported devices at once. Max Ad-Free removes the service’s commercials and allows streaming on two devices at once in Full HD. It also allows for 30 downloads at a time to allow users to watch content offline. On the other hand, Max Ultimate Ad-Free allows users to stream on four devices at once in a 4K Ultra HD resolution and provides Dolby Atmos audio and 100 downloads.

Frequently Asked Questions About Time Travel synopsis is as follows:

“Follows three social outcasts — two geeks and a cynic — as they attempt to navigate a time-travel conundrum in the middle of a British pub. Faris plays a girl from the future who sets the adventure in motion.”

NOTE: The streaming services listed above are subject to change. The information provided was correct at the time of writing.

Kristen Stewart: ‘It Sounds Like a F—Ing Nightmare’ Doing a Marvel Movie

Here release date set for robert zemeckis’ tom hanks-led drama, the woman in the yard: blumhouse removes release date for danielle deadwyler movie, orphan black: echoes trailer sets release date for krysten ritter spin-off.

A Detective Comics Universe fanatic and a media journalist, adept at covering the dynamic landscape of entertainment. Whether it’s exploring new worlds in a sci-fi novel, unraveling the mysteries of the superhero genre, or devising winning strategies in chess and video games, Dipesh is constantly seeking intellectual stimulation and creative inspiration.

Share article

Here Release Date Set for Robert Zemeckis’ Tom Hanks-Led Drama

Marvel and DC

Lashana Lynch Gives Update on Binary’s MCU Future

Dark Phoenix Director Simon Kinberg Talks Deadpool & Wolverine

Ghost Rider (2025): Is the Trailer Real or Fake? Will Keanu Reeves Play the MCU’s Johnny Blaze?

Head 2 Head Season 2 Streaming: Watch & Stream Online via HBO Max

Guy’s Grocery Games Season 15 Streaming: Watch & Stream Online via HBO Max

The Horror Crowd Streaming: Watch & Stream Online via Amazon Prime Video

Saturday Night Live Season 40 Streaming: Watch & Stream Online via Peacock